fix(trp): preserve RMI SMC X4 when not used as return

This patch adds X2-X6 and 'smc_ret' parameters to trp_rmi_handler().
The last 'smc_ret' parameter passed in X7 contains address of
'trp_smc_result' structure on stack to return result of RMI SMC call.

This allows to preserve X4 if not used as a return argument as per
SMCCCv1.2. The patch also removes use of trp_args_t in RMI handling.

Signed-off-by: AlexeiFedorov <Alexei.Fedorov@arm.com>
Change-Id: I9e3387a7380b37863eeccc53d13e92e0ac5cffbd