Docs: KVM: Rename psci.rst to hypercalls.rst

Since the doc also covers general hypercalls' details,
rather than just PSCI, and the fact that the bitmap firmware
registers' details will be added to this doc, rename the file
to a more appropriate name- hypercalls.rst.

Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Oliver Upton <oupton@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220502233853.1233742-6-rananta@google.com