arm64: zynqmp: Use zynqmp_mmio_read/write functions

Changed the return type of reset_reason() to int from u32, because
zynqmp_mmio_read/write() returns signed value on error.
Replaced readl and writel functions with zynqmp_mmio_read &
zynqmp_mmio_write functions to access RESET_REASON(CRL_APB) registers.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>