usb: host: xhci-plat: use ioremap_nocache

Use the ioremap_nocache variant of the ioremap API in
order to make sure our memory will be marked uncachable.

This patch should be backported to kernels as old as 3.4, that contain
the commit 3429e91a661e1f383aecc86c6bbcf65afb15c892 "usb: host: xhci:
add platform driver support".

Signed-off-by: Ruchika Kharwar <ruchika@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@vger.kernel.org