boards: arm: mps2_an521: Add TFM support

This commit adds support for TF-M to the MPS2 AN521.

When the CONFIG_BUILD_WITH_TFM flag is set, a secure and
non-secure processing environment image pair will be
generated, with the Zephyr application image running on
the non-secure side.

The secure and non-secure binary images will be signed
for use with the BL2 secure bootloader.

An additional .hex file is also generated to enable
running QEMU with the AN521 binaries, `tfm_qemu.hex`,
which can be executed with the `-t run` option with
west,  or `run` with ninja or make.

When configured for use with TF-M, the
`mps2_an521_nonsecure` board definition should be used.

Signed-off-by: Karl Zhang <karl.zhang@linaro.org>