[MIPS] Malta: remaining bits of the board support code cleanup

This patch factors out the code, which handles the Bonito system
controller. The case of not supporting the DMA coherency is handled
separately to make the logic obvious. Besides, a couple of empty
lines added to beautify the code even further.

No functional changes introduced.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>