The current implementation of snprintf() does not support pointer and
hexadecimal format specifiers, which can be needed, for instance, for
DTB manipulations.

This patch adds that functionality by borrowing some code from the
printf() implementation.

Signed-off-by: Javier Almansa Sobrino <javier.almansasobrino@arm.com>
Change-Id: I2076ea46693a73a04890982bf20e3c633c2767fb