arm64: debug: mark a function as __init to save some memory

'debug_monitors_init()' is only called via 'postcore_initcall'.
It can be marked as __init to save a few bytes of memory.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20200531110015.598607-1-christophe.jaillet@wanadoo.fr


Signed-off-by: Will Deacon <will@kernel.org>