firmware: zynqmp: Separate function for sending message via mailbox

U-Boot running in EL3 can't use SMC that's why there is a need to talk to
PMUFW directly via mailbox. The same logic is applied to all functions
which need to talk to PMUFW that's why move this logic to separate function
to avoid code duplication.

Also SMC request ID can be composed from PM_SIP_SVC offset that's why
ZYNQMP_SIP_SVC_GET_API_VERSION macro can be removed completely.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>