soc: renesas: Identify RZ/G2M

This patch adds support for identifying the RZ/G2M (r8a774a1) SoC.
It corrects the original RZ/G SoC family name to RZ/G1 and also
adds support for the new RZ/G2 SoC family.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Chris Paterson <chris.paterson2@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>