ARM HDLCD: Add useful functions to HDLCD driver for system integration

ARM HDLCD: Add useful functions to HDLCD driver for system integration

During TC2 integration a bad config option resulted in HDLCD memory reads
not being serviced often enough. This lead to unsightly screen blanking.

These options allow the developer to count the number of underruns and
to control the color used by HDLCD when an underrun prevents accessing
pixel data. The combination of these two options allow easy diagnosis
of HDLCD underrun conditions.

Signed-off-by: Chris Redpath <chris.redpath@arm.com>