hikey: fix build issue with CLANG

plat/hisilicon/hikey/hikey_bl1_setup.c:565:47:
error: value size does not match register size specified by the
constraint and modifier [-Werror,-Wasm-operand-widths]
        __asm__ volatile ("mrs  %0, cpacr_el1" : "=r"(data));

Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>