overlays: sc16is752-spi1: Add xtal parameter

The other sc16is75x overlays have an xtal parameter to allow a
different crystal frequency to be specified, but sc16is752-spi1
doesn't. Fix this omission.

See: https://www.raspberrypi.org/forums/viewtopic.php?f=107&t=273234



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