ARM: shmobile: r8a73a4: Add SoC-specific CMT compatible property

The CMT1 timer used the generic compatible property only.
Add the SoC-specific one, which is already documented, to make it future
proof.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>