rpi: use the newly-added RPI_EFI_NR_SPIN_PAGES

Some systems may use a slightly larger stub to do PSCI for booting the RPi
family.  The number of pages has been made configurable so that operating
systems building U-Boot for use in these kinds of environments can reserve
more memory in the EFI memory map.

Signed-off-by: Kyle Evans <kevans@FreeBSD.org>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>