firmware: zynqmp: get fw version with mailbox driver

Implements the function to get PMU Firmware version using the mailbox
driver or smc call based on if running SPL or not. Additionally gets
version as part of the ZynqMP Firmware driver probing

Signed-off-by: Ibai Erkiaga <ibai.erkiaga-elorza@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>