ARM: 7311/1: Add generic instruction opcode manipulation helpers

This patch adds some endianness-agnostic helpers to convert machine
instructions between canonical integer form and in-memory
representation.

A canonical integer form for representing instructions is also
formalised here.

Signed-off-by: Dave Martin <dave.martin@linaro.org>
Acked-by: Nicolas Pitre <nico@linaro.org>
Tested-by: Jon Medhurst <tixy@yxit.co.uk>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>