usb: phy: replace readb/writeb{_relaxed} with readl/writel{_releaxed}

USB QMP PHY registers are 32-bit. On some platform writeb_relaxed API
doesn't update device memory when register is 32-bit addressable. Hence
fix this issue by replacing readb/writeb{_relaxed} API with
readl/writel{_relaxed} APIs.

Change-Id: Iae1a938a7828189e7afcdef61860b6fcc487f7a8
Signed-off-by: Mayank Rana <mrana@codeaurora.org>