dt-bindings: timer: sifive,clint: Clean up compatible value section

Replace the sentences in the description listing some supported variants
by comments on the individual compatible values, to ease future
maintenance.  While at it, restore alphabetical sort order.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>