MIPS: bootm: add support for generic relocation of init ramdisks

All linux kernels after v2.6 require a page-aligned location of
an external init ramdisk. Enable CONFIG_SYS_BOOT_RAMDISK_HIGH to
support this with the generic U-Boot relocation code.

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