arm64: zynqmp: Use mailbox driver for PMUFW config loading

With new mailbox driver PMUFW configuration object can be loaded via the
same interface and there is no need to have pmu_ipc.c completely.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>