uboot-mediatek: rax3000m: use ddr3-1866mhz built in the dependency

uboot-mediatek: rax3000m: use ddr3-1866mhz built in the dependency

The u-boot of cmcc_rax3000m_*-ddr3 declares a dependency on
trusted-firmware-a-mt7981-*-ddr3-1866mhz, while actually packages
the normal BL2. This causes build failure if the the non-1866m
variant isn't built.
According to the previous commit, it's intended to use the lower
frequency variant to fix device stability issues. Correct
BL2_DDRTYPE as intended.

Fixes: 028050da69 ("mediatek: CMCC RAX3000Me: fix stability issues")
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>