MIPS: start.S: emulate REVISION register for qemu-malta

On the origial Malta boards the REVISION register is
accessible at the 0x1fc00010 address. The contents of
this register gives information about the revision
of the Malta and Core Boards.

This register is used by the Linux kernel to identify
the actual board it is running on. However the register
is not emulated properly by Qemu, so put a hardcoded
value into the flash to make Linux work.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>