ARM: mach-shmobile: Use DT_MACHINE for KZM9G

Use DT_MACHINE_START() on the sh73a0 based KZM9G board.

Also include a tiny DTS file to describe the board and
update the Kconfig dependencies to select CONFIG_USE_OF.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>