[POWERPC] 86xx: mpc8610_hpcd: fix second serial port

DIU platform code should not just write to the PIXIS' BRDCFG0 register,
it should set and clear its own bits only, otherwise it will break
firmware setup (in fact it breaks second uart).

Also get rid of magic numbers in the related code.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>