boards: mips: add Qemu Malta support

The MIPS Malta is an ATX form factor evaluation board made by MIPS
Technologies. Malta board is the most popular platform for MIPS
full-system emulation.

See https://www.linux-mips.org/wiki/MIPS_Malta

 for details.

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>