firmware: arm_ffa: Bound PARTITION_INFO_GET_REGS copies

firmware: arm_ffa: Bound PARTITION_INFO_GET_REGS copies

The register-based PARTITION_INFO_GET path trusted the firmware-provided
indices when copying partition descriptors into the caller buffer.
Reject inconsistent counts or index progressions so the copy loop cannot
write past the allocated array.

Fixes: ba85c644ac8d ("firmware: arm_ffa: Add support for FFA_PARTITION_INFO_GET_REGS")
Link: https://patch.msgid.link/20260428-ffa_fixes-v2-6-8595ae450034@kernel.org
(fixed cur_idx when exactly one descriptor in the first fragment)
Signed-off-by: Sudeep Holla <sudeep.holla@kernel.org>