net: zynq: gem: Use readl/writel_relaxed instead of __raw

For supporting ARM big-endian is necessary to use
proper IO endianess accessors.

Based on Ben Dooks BE guide.
Similar conversion is done here:
"mv_xor: use {readl, writel}_relaxed instead of __raw_{readl, writel}"
(sha1: 5733c38a)

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