spi/xilinx: Clean ioremap calling

devm_ioremap_resource() automatically checks that
struct resource is initialized.
Also group platform_get_resource() and devm_ioremap_resource()
together.
And remove mem resource from struct xilinx_spi.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Mark Brown <broonie@linaro.org>