ARM: OMAP2+: Incorrect Register Offsets in OMAP Mailbox

Looks like the register offsets are incorrect in the OMAP mailbox code
(arch/arm/mach-omap2/mailbox.c) for the OMAP4_MAILBOX_IRQ* macros. The
discrepancy is with p.224 of TI document SPRUGX9 and p3891 of SWPU231K.

Acked-by: Vaibhav Bedia <vaibhav.bedia@ti.com>
Signed-off-by: Henry Chan <enli.chan@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>