mips: mt7628: fix ddr_type for MT7688KN

The MT7688KN is a multi-chip package with 8MiB DDR1 KGD. So the DDR type
from bootstrap register must be ignored, and always be assumed as DDR1.

This patch fixes an issue that mt7628_ddr_pad_ldo_config() may be passed
with a wrong ddr_type in MT7688KN.

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