arm-trusted-firmware: Enable building in AGL build

Leverage the reworked extra_opl_ipt hook in the rcar3 BSP's
arm-trusted-firmware recipe as of BSP v4.7.0 to enable building the
reference hardware flavor of the firmware as an extra firmware
version when building with MACHINE=h3ulcb.  This will then enable
working inside the AGL build which combines the reference hardware
support into its h3ulcb build to avoid needing an extra platform
build.

Support for building standalone with MACHINE=agl-refhw-h3 has been
retained for potential use outside of the full AGL build.  The
firmware documentation has been reworked to document the build and
flashing procedure for both types of build.

Based on initial prototype by Vasyl Vavrychuk at:

https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-refhw/+/26352

Bug-AGL: SPEC-3916

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I5f74450851aeb4cb015a21e234709eca2014f378