BOOT: Add RAW ramdisk support to bootz

This patch allows loading RAW ramdisk via bootz command. The raw ramdisk is
loaded only in case it's size is specified:

  bootz <kernel addr> <ramdisk addr>:<ramdisk size> <fdt addr>

For example:

  bootz 0x42000000 0x43000000:0x12345 0x44000000

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Tom Warren <TWarren@nvidia.com>
Cc: albert.u.boot@aribaud.net
Cc: afleming@gmail.com
Cc: Simon Glass <sjg@chromium.org>
Cc: Stephen Warren <swarren@nvidia.com>
Cc: Nicolas Pitre <nico@fluxnic.net>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Detlev Zundel <dzu@denx.de>