USB: ehci-spear: Add PM support

This patch adds support for standby/S2R/hibernate for ehci-spear driver.

Signed-off-by: Deepak Sikri <deepak.sikri@st.com>
Signed-off-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>