MIPS: Add board_early_init_f() to init_sequence

This patch adds the board_early_init_f() call to the MIPS init
sequence. A weak dummy implementation is also added which can be
overridden by a board specific version.

This will be used by the upcoming VCTH board support.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>