arm: omap2plus: Enforce DM_SERIAL if SERIAL

With the migration deadline for DM_SERIAL, enforce DM_SERIAL if SERIAL
is enabled.  Continue to allow for non-DM_SERIAL in SPL.  This means in
some board header files we need to guard some defines for use in this
case.

Signed-off-by: Tom Rini <trini@konsulko.com>