lib/fonts: add font 6x8 for OLED display

This font is derived from lib/fonts/font_6x10.c and is useful for small
OLED displays

Signed-off-by: Sven Schneider <s.schneider@arkona-technologies.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200820082137.5907-1-s.hauer@pengutronix.de