metag: add exported <asm/ech.h> for extended context handling

Add an exported header file <asm/ech.h> containing the definitions of
some bits which can be set in D0.8 to indicate to the kernel that
certain DSP state should be preserved. The definitions have the same
names and values as the ones in the kernel-internal <asm/tbx.h>, to
make it easier for DSP assembly code to be compatible between Linux and
non-Linux operating systems.

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