ARM: shmobile: marzen: Switch to DT_MACHINE_START

Convert the marzen board to use DT_MACHINE_START. With this in
place all mach-shmobile boards use DT_MACHINE_START. Also, this
makes it possible for the r8a7779 SMP code to use DT for CPUs.

Signed-off-by: Magnus Damm <damm@opensource.se>
[horms+renesas@verge.net.au: Select USE_OF in Kconfig]
[horms+renesas@verge.net.au: Provide dt_compat in DT_MACHINE_START]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>