staging: mt7621-dts: use standard 'syscon' string

Both 'memc' and 'sysc' nodes are not using 'syscon'
as a node string which is the standard one to be used.
Update both of them.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20210505132154.8263-3-sergio.paracuellos@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>