firmware: arm_ffa: Fix big-endian support in __ffa_partition_info_regs_get()

Currently the FF-A driver doesn't support big-endian correctly. It is
hard to regularly test the setup due to lack of test infrastructure and
tools.

In order to support full stack, we need to take small steps in getting
the support for big-endian kernel added slowly. This change fixes the
support in __ffa_partition_info_regs_get() so that the response from the
firmware are converted correctly as required. With this change, we can
enumerate all the FF-A devices correctly in the big-endian kernel if the
FFA_PARTITION_INFO_REGS_GET is supported.

Tested-by: Viresh Kumar <viresh.kumar@linaro.org>
Message-Id: <20250217-ffa_updates-v3-5-bd1d9de615e7@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>