dt-bindings: timer: sifive,clint: add comaptibles for T-Head's C9xx

dt-bindings: timer: sifive,clint: add comaptibles for T-Head's C9xx

T-Head C906/C910 CLINT is not compliant to SiFive ones (and even not
compliant to the newcoming ACLINT spec) because of lack of mtime
register.

Add a compatible string formatted like the C9xx-specific PLIC
compatible, and do not allow a SiFive one as fallback because they're
not really compliant.

Signed-off-by: Icenowy Zheng <uwu@icenowy.me>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Samuel Holland <samuel@sholland.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230202072814.319903-1-uwu@icenowy.me
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>