feat(sptool): generate `ARM_BL2_SP_LIST_DTS` file from `sp_layout.json`

TF-A makefile accepts a device-tree snippet to override hardcoded SP
nodes, via the `ARM_BL2_SP_LIST_DTS` variable. However the SPs declared
in `ARM_BL2_SP_LIST_DTS` must be in the same order as they are in the
FIP image, otherwise hash authentication will fail when loaded by BL2.

This patch generates the `ARM_BL2_SP_LIST_DTS` file from the
`sp_layout.json` file. The SPs in the FIP image are also generated from
`sp_layout.json`, so this ensures that there is only one source of truth
for the SP list, removing the possibility to have the lists disagree
with each other.

Signed-off-by: Karl Meakin <karl.meakin@arm.com>
Change-Id: I7d76715135c596605c6a02aad5196d967dfeb1ce