remoteproc: omap: depend on OMAP_MBOX_FWK

Patch a62a6e98 "ARM: OMAP2+: Disable code that currently does not work
with multiplaform" makes the OMAP_MBOX_FWK option depend on !MULTIPLATFORM,
which means we cannot simply select that symbol from OMAP_REMOTEPROC.

Turning the 'select' into 'depends on' ensures that all dependencies
are correct until OMAP_MBOX_FWK loses its dependency.

Without this patch, building allmodconfig results in:

drivers/remoteproc/omap_remoteproc.c:31:26: fatal error: plat/mailbox.h: No such file or directory

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Ohad Ben-Cohen <ohad@wizery.com>