mips: mtmips: move mt7628 related Kconfig into mt7628 subdirectory

This patch is a preparation for add a new soc fot mtmips.

Move all mt7628 related Kconfig (boards and UART selection) into mt7628
subdirectory and make sure the top directory of mtmips contains only
selection for SoCs.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>