arm64: constify aarch64_insn_encoding_class[]

The aarch64_insn_encoding_class[] array contains compile-time constant
data, and is never intentionally modified, so let's mark it as const.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>