ARM: shmobile: kzm9g-reference: Remove board C code and DT file

Now that the sh73a0 generic multiplatform case has the same feature set
as the kzm9g DT reference board code, we get rid of the latter.
DT reference code in the future shall make use of the sh73a0
multiplatform support code with the generic SoC machine vector.

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