omap: mailbox: correct OMAP4 reset logic

OMAP4 mailbox has a different register set. There is
no MAILBOX_SYSSTATUS register. The reset is indicated
with the SOFTRESET bit of the MAILBOX_SYSCONFIG register
itself. This bit should read 0 for a successful Reset.
Also, the SOFTRESET bit occupies bit0 and not bit1 as with
previous generations.

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>