arm64: v8.3: Support for Javascript conversion instruction

ARMv8.3 adds support for a new instruction to perform conversion
from double precision floating point to integer  to match the
architected behaviour of the equivalent Javascript conversion.
Expose the availability via HWCAP and MRS emulation.

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

Documentation/arm64/cpu-feature-registers.txt

@@ -169,6 +169,14 @@ infrastructure:
    as available on the CPU where it is fetched and is not a system
    wide safe value.
 
+  4) ID_AA64ISAR1_EL1 - Instruction set attribute register 1
+
+     x--------------------------------------------------x
+     | Name                         |  bits   | visible |
+     |--------------------------------------------------|
+     | JSCVT                        | [15-12] |    y    |
+     x--------------------------------------------------x
+
 Appendix I: Example
 ---------------------------