configs: set CONFIG_LMB_MAX_REGIONS=64 for MT7988 boards

Similar to MT7981 and MT7986 also MT7988 can have a high number of
reserved-memory regions used by the various hardware offloading
subsystems.

Raise CONFIG_LMB_MAX_REGIONS to 64 to avoid errors when trying to boot
Linux with more then 6 reserved regions:

ERROR: reserving fdt memory region failed (addr=4f700000 size=240000 flags=4)
ERROR: reserving fdt memory region failed (addr=15194000 size=1000 flags=4)
ERROR: reserving fdt memory region failed (addr=15294000 size=1000 flags=4)
ERROR: reserving fdt memory region failed (addr=15394000 size=1000 flags=4)
ERROR: Failed to allocate 0xb161 bytes below 0x80000000.
device tree - allocation error

Fixes: bc4adc97 ("board: mediatek: add MT7988 reference boards")
Reported-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>