soc: qcom: scm: fix boot issue when compiled with clang

As per SMC Calling Convention, X4/W4, X5/W5 & X6/W6 registers
return state is unpredictable. But, inline assembly in scm
driver did not correctly specify output markers, which is
causing compiler to move instructions around when clang is
used to compile, which inturn causing deadlock and boot issue
during firmware loading. Add X4/W4, X5/W5 & X6/W6 registers
to output operand list to fix the deadlock during firmware
loading.

Change-Id: I1e4d51a58cd34c31fc63bcd21a20bfd15498bdc3
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>