xilinx: zynqmp: use zynqmp_mmio_read() in multi_boot()

When U-Boot runs in EL2 there is no access to csu_base registers that's why
this has to be done via firmware interface to find out multi boot register
value. Till now this function is called only from SPL in EL3.

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