Set console function pointers to const

Set the function pointers in the console struct and the functions
they point to to const since they only need to be defined when
the console is being initialised and should not be changed after

Change-Id: I0574307111e3ab2f13d1a4a74c3fa75532dfa4be
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>