metag: Boot

Add boot code for metag. Due to the multi-threaded nature of Meta it is
not uncommon for an RTOS or bare metal application to be started on
other hardware threads by the bootloader. Since there is a single MMU
switch which affects all threads, the MMU is traditionally configured by
the bootloader prior to starting Linux. The bootloader passes a
structure to Linux which among other things contains information about
memory regions which have been mapped. Linux then assumes control of the
local heap memory region.

A kernel arguments string pointer or a flattened device tree pointer can
be provided in the third argument.

Signed-off-by: James Hogan <james.hogan@imgtec.com>