[ARM] Fix Versatile PB initialisation to use .init_machine

There's no point having the Versatile PB initialisation using an
arch_initcall() and then checking whether it's running on a
Versatile PB board - this is what the .init_machine function
pointer in the machine description is for.  Use it.

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