MAINTAINERS: exclude defconfig from ARM64 PORT

Patches for the arm64 defconfig are supposed to be sent to the
SoC maintainers (e.g. a change in the generic arm64 defconfig
required for Rockchip devices should be send to Heiko Stübner
as he is listed as maintainer for "ARM/Rockchip SoC support")
and not the ARM64 PORT maintainers.

While we cannot easily describe this in MAINTAINERS, we can at
least stop it from giving false information and make it behave
the same way as for the MAINTAINERS file itself (which basically
has the same rules), so that it just outputs the LKML for the
ARM64 defconfig.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20250818-arm64-defconfig-v1-1-f589553c3d72@collabora.com
Signed-off-by: Arnd Bergmann <arnd@arndb.de>