ARM: shmobile: Add basic RZ/A2 SoC support

Add the RZ/A2 SoC to the Renesas SoC collection.

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>