ARM: simplify early machine init hooks

Rather than storing each machine init hook separately, store a
pointer to the machine description record and dereference this
instead.  This pointer is only available while the init sections
are present, which is not a problem as we only use it from init
code.

Change-Id: Ib86b2a6c9fb8e5a9e933f2815b6186b211974500
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>