ARM: OMAP2 Provide function to enable/disable uart clocks

This patch adds common function to enable/disable omap2/3 uart
clocks. Enabled uarts are passed by bootloader in atags and clocks for
these enabled uarts are touched.

Signed-off-by: Jouni Hogander <jouni.hogander@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>