When compiling with android toolchain, there is an instruction
"str     q0, [x8],#16

", but x8 is not 16bytes aligned,
this instruction will trigger sync abort.

So, following Linux kernel, only use general regs for arm64.
If not, compiler may use simd registers Q[x]. We need to avoid
using simd registers in U-Boot, because load/store Q[x] has
restriction that 128bits aligned when str/ldr.

Signed-off-by: Peng Fan <peng.fan@nxp.com>