tf_snprintf: Add support for '%s' ...

Change-Id: Ia3a159444e638f63de7dc5a6a4b76169c757188a
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>

Introduce `tf_snprintf` ...

This is a reduced version of `snprintf` that only supports formats '%d',
'%i' and '%u'. It can be used when the full `snprintf` is not needed in
order to save memory. If it finds an unknown format specifier, it
prints an error message and panics.

Change-Id: I2cb06fcdf74cda2c43caf73ae0762a91499fc04e
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>