sysutils/u-boot-rpi{3,4}: Add patch to fix PSCI stub reservation

For the traditional spin table setup, only the lowest page needs
to be reserved. However, our PSCI stubs are slightly bigger and occupy
a little over one page. These patches, already sent upstream, allow a
variable number of initial pages to be reserved for the SMP stubs. We
then set the values for these in our fragment to '2' to reserve the first
and second page.

Approved by: koobs (mentor)
Approved by: manu (uboot, maintainer)
MFH: 2020Q1
Differential Revision: https://reviews.freebsd.org/D24085