mpc5200: digsy_mtc: add support for writing 'appreg' value

Up to now only reading 'appreg' value was implemented in the
digsyMTC special 'mtc appreg' command. Extend the command to
support writing appreg value, too.

Signed-off-by: Werner Pfister <Pfister_Werner@intercontrol.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Detlev Zundel <dzu@denx.de>