aarch64: add big endian(aarch64_be) support

Add aarch64_be support. Note that CONFIG_CPU_BIG_ENDIAN should be
defined in kernel config when building a big endian kernel.

Signed-off-by: Zhang Jian(Bamvor) <bamvor.zhangjian@huawei.com>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>