OMAP: mailbox: use runtime pm for clk and sysc handling

Use runtime pm APIs to enable/disable mailbox clocks and
to configure SYSC register.

Based on the patch sent by Felipe Contreras:
https://patchwork.kernel.org/patch/101662/

Signed-off-by: Omar Ramirez Luna <omar.ramirez@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>