ppc32: Fix BCSR_SIZE for MPC834x SYS

According to the 'MPC8349E MDS Processor Board User Manual Rev. 1.6'
the size of the BCSR mapping is 32kb.

Signed-off-by: Horst Kronstorfer <hkronsto@frequentis.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>