Platform: Add LPCXpresso55S69-EVK support

This commit adds support for the ARM Cortex-M33 based
LPCXpresso55S69-EVK. It is based on NXP's internal SDK, which
includes a version of TF-M with a custom build system, making
the NXP SDK code compatible with the upstream TF-M build system.

The following changes were required outside of the platform/ext
folder:

- `LPC55S69` was added as a target platform to the config files.

Notes on working with TF-M and the LPCXpresso55S69-EVK are available
in `platform/ext/target/nxp/lpcxpresso55s69/README.rst`.

Task ID: https://developer.trustedfirmware.org/T706


Author: Andrej Butok <andrey.butok@nxp.com>
Co-authored-by: Kevin Townsend <kevin.townsend@linaro.org>
Signed-off-by: Kevin Townsend <kevin.townsend@linaro.org>
Change-Id: I998cd83abdcd4919d102a3e12cc7c3eb17450b99