metag: Turn irq_ctx_* macros into static inlines

The irq_ctx_init()/irq_ctx_exit() functions are only used for 4KiB
stacks, otherwise they are implemented as an empty macro. To prevent
future bit rottage due to the cpu argument not being type checked,
convert the macros to empty static inline functions.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-metag@vger.kernel.org