Fix this:

arch/arm/mach-omap2/mailbox.c: In function 'omap2_mbox_probe':
arch/arm/mach-omap2/mailbox.c:354: error: 'omap2_mboxes' undeclared (first use in this function)
arch/arm/mach-omap2/mailbox.c:354: error: (Each undeclared identifier is reported only once
arch/arm/mach-omap2/mailbox.c:354: error: for each function it appears in.)

Which happens on CONFIG_ARCH_OMAP2 && !CONFIG_SOC_OMAP2420, due to
missing omap2_mboxes declaration.

In addition, make sure we declare the right mailbox instances for 2430.
Reported-by: NRussell King <linux@arm.linux.org.uk>
Signed-off-by: NOhad Ben-Cohen <ohad@wizery.com>
Cc: Hiroshi Doyu <hdoyu@nvidia.com>
Cc: Omar Ramirez Luna <omar.ramirez@ti.com>
Signed-off-by: NTony Lindgren <tony@atomide.com>