arm: make memory power routines conform to current generic API

The various routines to change memory power state used
in physical memory hotplug and hotremove used to take
a start pfn and a number of pages and return 1 for success
and 0 for failure.

The generic API these are called from now takes a start address
and size and returns a byte count of memory powered on or
off, so the ARM and platform specific routines should as well.

Signed-off-by: Larry Bassel <lbassel@codeaurora.org>