Functions provided by stdio.h such as printf and sprintf are available
in the codebase, but they add a lot of code to the final image if they
are used:

- AArch64: ~4KB
- AArch32: ~2KB in T32, ~3KB in A32

tf_printf and tf_snprintf are a lot more simple, but it is preferable
to use them when possible because they are also used in common code.

Change-Id: Id09fd2b486198fe3d79276e2c27931595b7ba60e
Acked-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>