SCP clock is needed for CIO on OMAP3, and for CIO and PLL on OMAP4.
Current driver enables the CIO clock always when DSI display is
initialized. However, if a DPI display tries to use DSI PLL, the SCP
clock is never enabled.

This patch implements simple ref counting enable/disable functions for
SCP clock.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@ti.com>