ARM: dts: alt: Fix PFC names for DU

Update the PFC pin groups and function names of DU interface for
r8a7794 ALT board.

The currently specified pin groups and function names prevented PFC and
DU interfaces from being correctly configured:

sh-pfc e6060000.pin-controller: function 'du' not supported
sh-pfc e6060000.pin-controller: invalid function du in map table
sh-pfc e6060000.pin-controller: function 'du' not supported
sh-pfc e6060000.pin-controller: invalid function du in map table
sh-pfc e6060000.pin-controller: function 'du' not supported
sh-pfc e6060000.pin-controller: invalid function du in map table
sh-pfc e6060000.pin-controller: function 'du' not supported
sh-pfc e6060000.pin-controller: invalid function du in map table
rcar-du: probe of feb00000.display failed with error -22

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>