drivers: phy: exynos-usb2: add support for Exynos 3250

drivers: phy: exynos-usb2: add support for Exynos 3250

This patch adds support for Exynos3250 SoC to Exynos2USB PHY driver.
Although Exynos3250 has only one device phy interface, the register
layout and all operations that are required to get it enabled are almost
same as on Exynos4x12. The only different is one more register
(REFCLKSEL) which need to be set and lack of MODE SWITCH register.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>