ARM: rmobile: beacon: Support RZ/G2 [M/N/H] on TARGET_BEACON_RZG2M

The three different boards share the same board file and only differ
in terms of which SoC is being used.  By enabling FIT on
TARGET_BEACON_RZG2M, it can support all three boards and elimate
duplicate code.

Signed-off-by: Adam Ford <aford173@gmail.com>