usb: phy: samsung: Select common driver part implicitly

Since phy-samsung-usb library can be used only by phy-samsung-usb2 and
phy-samsung-usb3 drivers, there is no need to give explicit control over
its Kconfig symbol.

This patch makes CONFIG_SAMSUNG_USBPHY symbol hidden and selected
implicitly by CONFIG_SAMSUNG_USB2PHY and CONFIG_SAMSUNG_USB3PHY.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>