arm64: move encode_insn_immediate() from module.c to insn.c

Function encode_insn_immediate() will be used by other instruction
manipulate related functions, so move it into insn.c and rename it
as aarch64_insn_encode_immediate().

Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Jiang Liu <liuj97@gmail.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>