samples: tfm: PSA firmware update sample

This commit adds a sample application demonstrating how to use
the PSA Firmware Update API from TF-M. It also enables the
`FIRMWARE_UPDATE` partition to be included at build time.

Signed-off-by: Kevin Townsend <kevin.townsend@linaro.org>
Signed-off-by: Jimmy Brisson <jimmy.brisson@linaro.org>