coresight: Only check bottom two claim bits

coresight: Only check bottom two claim bits

The use of the whole register and == could break the claim mechanism if
any of the other bits are used in the future. The referenced doc "PSCI -
ARM DEN 0022D" also says to only read and clear the bottom two bits.

Use FIELD_GET() to extract only the relevant part.

Reviewed-by: Leo Yan <leo.yan@arm.com>
Reviewed-by: Yeoreum Yun <yeoreum.yun@arm.com>
Signed-off-by: James Clark <james.clark@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20250325-james-coresight-claim-tags-v4-2-dfbd3822b2e5@linaro.org