spl: atf: Fix clang -Wasm-operand-widths warning

spl: atf: Fix clang -Wasm-operand-widths warning

common/spl/spl_atf.c:187:51: warning: value size does not match register
  size specified by the constraint and modifier [-Wasm-operand-widths]
    __asm__ __volatile__("msr DAIF, %0\n\t" : : "r" (daif) : "memory");
                                                     ^
common/spl/spl_atf.c:187:34: note: use constraint modifier "w"
    __asm__ __volatile__("msr DAIF, %0\n\t" : : "r" (daif) : "memory");
                                    ^~
                                    %w0

Use %x0 to match what Linux does in <asm/sysreg.h> write_sysreg().

Signed-off-by: Alistair Delva <adelva@google.com>
Cc: Kever Yang <kever.yang@rock-chips.com>
Cc: Michael Walle <michael@walle.cc>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>