MIPS: qemu_mips: Import asm/unaligned.h from the Linux kernel

with a few adjustments for U-Boot.  This fixes the following build error:

make -C lib_generic/
zlib.c:31:27: error: asm/unaligned.h: No such file or directory
zlib.c: In function 'inflate_fast':
zlib.c:641: warning: implicit declaration of function 'get_unaligned'
make[1]: *** [zlib.o] Error 1
make[1]: Leaving directory `/home/skuribay/git/u-boot.git/lib_generic'
make: *** [lib_generic/libgeneric.a] Error 2

Reported-by: Himanshu Chauhan <himanshu@symmetricore.com>
Signed-off-by: Shinya Kuribayashi <skuribay@pobox.com>