omap: mailbox: correct OMAP4 SIDLEMODE logic

OMAP4 mailbox has a different MAILBOX_SYSCONFIG register.
There is now no AutoIdle. The SIDLEMODE bits have been
right-shifted by 1 bit and now occupy bits 2 & 3.

Signed-off-by: Suman Anna <s-anna@ti.com>
Acked-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>