drivers: display: mb_display: Add empty column to scrolling text

Add an empty column between characters of scrolling text so that the
individual characters are more easily distinguishable instead of being
back-to-back mashed together.

Also adjust the default scrolling step interval so that the character
display frequency stays roughly the same as before.

Change-Id: Idca0dc149a84f3f99b753a28ad1120ce75b97667
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>