OMAP2: fix recent build breakage

Fix recent build breakage.  Boot-tested on N800.

  CC      arch/arm/mach-omap2/devices.o
arch/arm/mach-omap2/devices.c:110: error: 'OMAP34XX_MAILBOX_BASE' undeclared here (not in a function)
arch/arm/mach-omap2/devices.c:115: error: 'INT_34XX_MAIL_U0_MPU' undeclared here (not in a function)
make[1]: *** [arch/arm/mach-omap2/devices.o] Error 1

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>