lib/printf: Add support for printing wide strings

This change adds support for wide strings (u16*) to printf()
variants. This feature is used by a future change.

Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Ricardo Koller <ricarkol@google.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>