mediatek: use consistent naming scheme for device nodes

mediatek: use consistent naming scheme for device nodes

This harmonizes the device node names (and thus the image names, too)
between subtargets of the mediatek target. So far, each subtarget
has somewhat used its own naming scheme. Now, we use the vendor_device
syntax there, too.

Since DTS names have different patterns and the target only contains
a few devices, this does not replace DEVICE_DTS by a calculated
default value (like for other targets).

SUPPORTED_DEVICES is adjusted based on the node rename where necessary,
though it looks like for several older devices it was not set up
correctly so far.

While at it, this also changes the DTS name for u7623-02-emmc-512m
to all-lower-case.

Cc: John Crispin <john@phrozen.org>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>