bootm: fix conditional compilation for bootm ramdisk subcommand

All code related to the bootm ramdisk subcommand is conditionally
enabled by CONFIG_SYS_BOOT_RAMDISK_HIGH except for the help message.
Replace the CONFIG_ARCH defines by CONFIG_SYS_BOOT_RAMDISK_HIGH
to fix this.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>