mips: vct: move CONFIG_SYS_TEXT_BASE away from config.mk

We can delete board/micronas/vct/config.mk by moving the define of
CONFIG_SYS_TEXT_BASE to somewhere else.  Other MIPS boards are
still defining it in include/config/${BOARD}.h, so I am following
that way here.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>