drivers: display: Enhance BBC micro:bit string output capabilities

Add a new mb_display_print() API which takes printf-style parameters
and outputs scrolling text instead of one character at a time. The
existing mb_display_str() API is renamed to mb_display_string() for
consistency, and now also takes printf-style parameters.

Change-Id: I59c42bcd74c62f05ecb6d097dc808b9e5c1984c5
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>