MdeModulePkg/ArmFfaLib: support ArmFfaLib wihtout Rx/Tx buffer

In the normal world, it is possible to communicate with
a secure partition using the ARM_FFA_PARTITION_INFO_GET_REGS ABI,
even when the Rx/Tx buffer ABI is not supported.

Therefore, treat the EFI_UNSUPPORTED error returned
during Rx/Tx buffer mapping as a valid result
when the ARM_FFA_PARTITION_INFO_GET_REGS ABI is supported.

Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>