MIPS: kconfig: add options for vct board variant select

The vct board can be built with various variants which are
configured via CONFIG_SYS_EXTRA_OPTIONS. This is deprecated. Thus
add new Kconfig options for those board variants.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Acked-by: Stefan Roese <sr@denx.de>