omap3: serial: Add context save and restore for mcr

Adds context save/restore for mcr register as state of mcr register
is lost after core off.

Signed-off-by: Govindraj R <govindraj.raja@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>