dt-bindings: mmc: starfive: Remove properties from required

dt-bindings: mmc: starfive: Remove properties from required

Due to the change of tuning implementation, it's no longer necessary to
use the "starfive,sysreg" property in dts, so remove it from required.

Signed-off-by: William Qiu <william.qiu@starfivetech.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Link: https://lore.kernel.org/r/20230922062834.39212-2-william.qiu@starfivetech.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>