metag: make TXPRIVEXT bits explicit

Define PRIV_BITS using explicit constants from <asm/metag_regs.h> rather
than with a hard coded value. This also adds a couple of missing
definitions for the TXPRIVEXT priv bits for protecting writes to TXTIMER
and the trace registers.

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