asm-generic: simplify unaligned.h

The get_unaligned()/put_unaligned() implementations are more
complex than necessary.

Move everything into one file and use a more compact implementation based
on packed struct access and byte swapping macros.

This patch is based on the Linux kernel commit 803f4e1eab7a
("asm-generic: simplify asm/unaligned.h") by Arnd Bergmann.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>