ARM: OMAP1: Fix build related to kgdb.h no longer including serial_8250.h

Commit 16559ae4

 (kgdb: remove #include <linux/serial_8250.h> from kgdb.h)
had a side effect of breaking omap1_defconfig build as some headers
were included indirectly:

arch/arm/mach-omap1/board-h2.c:249: error: ‘INT_KEYBOARD’ undeclared here (not in a function)
...

This worked earlier as linux/serial_8250.h included linux/serial_core.h,
via linux/serial_8250.h from linux/kgdb.h. Fix this by including the
necessary headers directly.

Signed-off-by: Tony Lindgren <tony@atomide.com>