overlays: Fix sc16is75x overlays w.r.t. serdev

Enabling serdev support in rpi-5.4.y had the unintended consequence of
making any UART device node with a subnode look like a "serdev" node,
which prevents it from having the usual /dev/ttyXXX character device.
Solve the problem by moving the subnode (a static clock declaration)
into the root node.

At the same time, regularise (and sometimes correct) the overlays.

See: https://github.com/raspberrypi/linux/issues/3765



Signed-off-by: Phil Elwell <phil@raspberrypi.com>