clk: meson: meson8b: remove compatibility code for old .dtbs

The XTAL clock is provided via .dts since Linux 5.6. Remove
compatibility code for .dtbs which are older than that.

The switch to the HHI syscon has been done with Linux 5.1. Also remove
any code needed to support .dtbs that have not switched to the HHI
syscon yet.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>