powerpc/p2041rdb: set sysclk according to status of physical switch SW1

P2041RDB supports 3 sysclk frequencies, it's selected by SW1[6~8],
software need to read the SW1 status to decide what the sysclk needs.

SW1[8~6] : frequency
0 0 1 : 83.3MHz
0 1 0 : 100MHz
others: 66.667MHz

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>