ARM: pxa: enable pxafb unconditionally for some boards

The SAAR and TAVOREVB machines try to call functions from
the PXAFB frame buffer driver from their platform code,
which only works if that driver is built-in.

This patch ensures that both the generic frame buffer
code and the specific pxafb driver are always enabled
when we build a kernel for one of the two boards.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Eric Miao <eric.y.miao@gmail.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Cc: Daniel Mack <zonque@gmail.com>