ARM: shmobile: sh73a0: add PMU information to sh73a0.dtsi

Add PMU information to sh73a0.dtsi. With this
included KZM9G DT reference may use the PMU.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>