arm64/capabilities: Make use of system wide safe value

Now that we can reliably read the system wide safe value for a
feature register, use that to compute the system capability.
This patch also replaces the 'feature-register-specific'
methods with a generic routine to check the capability.

Signed-off-by: Suzuki K. Poulose <suzuki.poulose@arm.com>
Tested-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/cpufeature.h

@@ -78,6 +78,7 @@ struct arm64_cpu_capabilities {
 		};
 
 		struct {	/* Feature register checking */
+			u32 sys_reg;
 			int field_pos;
 			int min_field_value;
 		};