ARM: mach-shmobile: kzm9g: add zboot support

Adds support to run the kernel on the uninitialized KZM9G board, using for
instance the mask ROM boot loader or JTAG. This patch tries to emulate the
style of the corresponding "mackerel" implementation. The DRAM controller
setup code has been adapted from u-boot.

Signed-off-by: Ulrich Hecht <ulrich.hecht@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>