rpi: Kconfig option for initial page reservation

While the nearly-universal default for the Raspberry Pi family is to use
spin tables and the spin table implementation provided by the Raspberry Pi
Foundation, FreeBSD and others may use a PSCI implementation instead.

Accommodate these setups by allowing them to configure for more than one
page to be reserved in the initial reservation.  The default reservation
remains as one page.

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