dev-arm: Add a fake SP810 to the VExpress_GEM5_Foundation

dev-arm: Add a fake SP810 to the VExpress_GEM5_Foundation

The device is part of the FVP Foundation Platform:

https://static.docs.arm.com/100961/1190/armv8_a_fp_ug_100961_1190_00_en.pdf

Change-Id: I81c11312f29d8e59ac5f8ce2fe165d9474027d82
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39696
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>