GPIO: xilinx: Use __raw_readl/__raw_writel IO functions

This driver can be used on Xilinx ARM Zynq platform
where in_be32/out_be32 functions are not implemented.
Use __raw_readl/__raw_writel functions which are
implemented on Microblaze and PowerPC.
For ARM readl/writel functions are used instead.

The correct way how to implement this is to detect
endians directly on IP. But for the gpio case
without interrupt connected(it means without
interrupt logic) there are just 2 registers
data and tristate where auto detection can't be done.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>