MIPS: Add CONFIG_SKIP_LOWLEVEL_INIT

This patch adds the CONFIG_SKIP_LOWLEVEL_INIT option to start.S. This
enables support for boards where the lowlevel initialization is
already done when U-Boot runs (e.g. via OnChip ROM).

This will be used in the upcoming VCTH board support.

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