powerpc: increase MIN RMA size for CAS negotiation

Change RMA size from 512 MB to 768 MB which will result
in more RMA at boot time for PowerPC. When PowerPC LPAR use/uses vTPM,
Secure Boot or FADump, the 512 MB RMA memory is not sufficient for
booting. With this 512 MB RMA, GRUB2 run out of memory and unable to
load the necessary. Sometimes even usage of CDROM which requires more
memory for installation along with the options mentioned above troubles
the boot memory and result in boot failures. Increasing the RMA size
will resolves multiple out of memory issues observed in PowerPC.

Failure details:

1. GRUB2

kern/ieee1275/init.c:550: mm requested region of size 8513000, flags 1
kern/ieee1275/init.c:563: Cannot satisfy allocation and retain minimum runtime
space
kern/ieee1275/init.c:550: mm requested region of size 8513000, flags 0
kern/ieee1275/init.c:563: Cannot satisfy allocation and retain minimum runtime
space
kern/file.c:215: Closing `/ppc/ppc64/initrd.img' ...
kern/disk.c:297: Closing
`ieee1275//vdevice/v-scsi
@30000067/disk@8300000000000000'...
kern/disk.c:311: Closing
`ieee1275//vdevice/v-scsi
@30000067/disk@8300000000000000' succeeded.
kern/file.c:225: Closing `/ppc/ppc64/initrd.img' failed with 3.
kern/file.c:148: Opening `/ppc/ppc64/initrd.img' succeeded.
error: ../../grub-core/kern/mm.c:552:out of memory.

2. Kernel

[ 0.777633] List of all partitions:
[ 0.777639] No filesystem could mount root, tried:
[ 0.777640]
[ 0.777649] Kernel panic - not syncing: VFS: Unable to mount root fs on "" or unknown-block(0,0)
[ 0.777658] CPU: 17 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.11.0-0.rc4.20.el10.ppc64le #1
[ 0.777669] Hardware name: IBM,9009-22A POWER9 (architected) 0x4e0202 0xf000005 of:IBM,FW950.B0 (VL950_149) hv:phyp pSeries
[ 0.777678] Call Trace:
[ 0.777682] [c000000003db7b60] [c000000001119714] dump_stack_lvl+0x88/0xc4 (unreliable)
[ 0.777700] [c000000003db7b90] [c00000000016c274] panic+0x174/0x460
[ 0.777711] [c000000003db7c30] [c00000000200631c] mount_root_generic+0x320/0x354
[ 0.777724] [c000000003db7d00] [c0000000020066f8] prepare_namespace+0x27c/0x2f4
[ 0.777735] [c000000003db7d90] [c000000002005824] kernel_init_freeable+0x254/0x294
[ 0.777747] [c000000003db7df0] [c00000000001131c] kernel_init+0x30/0x1c4
[ 0.777757] [c000000003db7e50] [c00000000000debc] ret_from_kernel_user_thread+0x14/0x1c
[ 0.777768] --- interrupt: 0 at 0x0
[ 0.784238] pstore: backend (nvram) writing error (-1)
[ 0.790447] Rebooting in 10 seconds..

Signed-off-by: Avnish Chouhan <avnish@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250123114254.200527-4-sourabhjain@linux.ibm.com