usb: phy: mxs: use readl(), writel() instead of the _relaxed() versions

This patch converts the mxs-phy driver from readl_relaxed(), writel_relaxed()
to the plain readl(), writel() functions, which are available on all platforms.

This is done to enable compile time testing on non ARM platforms.

Reported-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>