[MIPS] Rework cobalt_board_id

This patch has reworked cobalt_board_id.
The cobalt_board_id is read from PCI config register.
It should be in PCI routine.

Signed-off-by: Yoichi Yuasa <yoichi_yuasa@tripeaks.co.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>