[][kernel][mt7988][bsp][Change number of CPUs to 3 for the MT7988C/D]

[Description]
Change number of CPUs to 3 for the MT7988C/D.

Without this patch, the CPU will waste time waiting for a non-existent
CPU to be brought up during the kernel bootup procedure.

[Release-log]
N/A


Change-Id: I5332364700fb075a4e86fb384c6d29aec423fac0
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/7748718
8 files changed