staging: mt7621-pci-phy: convert driver to use kernel regmap API's

Instead of using writel and readl use regmap API which makes
the driver maintainability easier.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>