metag: Headers for core arch constants

Add a couple of header files containing core architecture constants.

The first (<asm/metag_isa.h>) contains some constants relating to the
instruction set, such as values to give to the CACHEW and CACHER
instructions.

The second (<asm/metag_regs.h>) contains constants for the core register
units directly accessible to various instructions, and for the
registers, fields, and bits in those units. The main units described are
the control unit (CT.*), the trigger unit (TR.*), and the run-time trace
unit (TT.*).

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