arm64: Advertise support for Rounding double multiply instructions

ARM v8.1 extensions include support for rounding double multiply
add/subtract instructions to the A64 SIMD instructions set. Let
the userspace know about it via a HWCAP bit.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/cpuinfo.c

@@ -64,6 +64,7 @@ static const char *const hwcap_str[] = {
 	"fphp",
 	"asimdhp",
 	"cpuid",
+	"asimdrdm",
 	NULL
 };