overlays: Fix multiple-instantiation of sc16is7xx*

overlays: Fix multiple-instantiation of sc16is7xx*

The registration of the fixed clocks uses the node name as the clock
name, causing a clash if two clock nodes have the same name, regardless
of the path to the node. Fix the issue by overwriting the clock node
names using the value of the "addr" parameter, providing a crude
disambiguation. (A bit of string pasting to form "sc16is752_clk_<addr>"
would have been nice, but that is outside the abilities of the overlay
parameter mechanism.)

Also give the sc16is750-i2c overlay the xtal parameter for symmetry.

See: https://www.raspberrypi.org/forums/viewtopic.php?f=107&t=235650

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