[MIPS] Altas, Malta: Switch boot file format to raw.

A raw binary boots about twice as fast as SREC.

The possibility to generate SREC binaries remains by simply using the
vmlinux.srec target but seems only useful for the probably hypothetical
case where one of these systems is booted over a serial interface.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>