modules: loramac-node: fix CN470 linking

Update the source files compiled when `CONFIG_LORAMAC_REGION_CN470` is
enabled to link. Despite the naming, a `RegionBaseUS.c` function
(`RegionBaseUSVerifyFrequencyGroup`) is used by all four of the CN470
band implementations.

Validated by compiling `samples/subsys/lorawan/class_a` with
`CONFIG_LORAMAC_REGION_CN470` instead of `CONFIG_LORAMAC_REGION_IN865`.

Fixes #49960.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>