EXYNOS5: Add function to enable exynos5420 usbdev phy ctrl

Exynos5420 has different registers with other exynos5 SoCs to control
usb device phy, so need separated function to enable exynos5420 usb
device phy.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>