ARM: OMAP2+: Fix unmet direct dependencies for SERIAL_OMAP

Commit 8a6201b9 (ARM: OMAP2+: Fix unmet direct dependencies for zoom
for 8250 serial) fixed unmet direct dependencies for 8250, but failed
to do the same for omap serial. This can cause the following warning:

warning: (ARCH_OMAP2PLUS_TYPICAL) selects SERIAL_OMAP which has
unmet direct dependencies (TTY && HAS_IOMEM && GENERIC_HARDIRQS &&
ARCH_OMAP2PLUS).

We should not select drivers, they should be selected by the
user. Fix the issue by removing the select and adding them to
omap2plus_defconfig instead.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>