SPI: zynq: Use readl_relaxed and writel_relaxed

Use readl_relaxed and writel_relaxed because
it has the advantage of taking care of endian-ness.
and also as raw_readl and raw_writel are not recommended
according to the below thread:
http://comments.gmane.org/gmane.linux.ports.arm.kernel/117626



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