EXYNOS: Add power Enable/Disable for USB-EHCI

This patch adds functions to enable/disable the power of USB
host controller for EXYNOS5.

Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Signed-off-by: Che-Liang Chiou <clchiou@chromium.org>
Signed-off-by: Rajeshwari Shinde <rajeshwari.s@samsung.com>