firmware: qcom-scm: build for correct architecture level

The ".arch_extension sec" directive is only available on ARMv6 or higher,
so if we enable the SCM driver while building a kernel for an older CPU,
we get a build error:

/tmp/ccUyhMOY.s:130: Error: selected processor does not support ARM mode `smc #0'
/tmp/ccUyhMOY.s:216: Error: selected processor does not support ARM mode `smc #0'
/tmp/ccUyhMOY.s:373: Error: selected processor does not support ARM mode `smc #0'
make[4]: *** [drivers/firmware/qcom_scm-32.o] Error 1

This changes the Makefile so we pass the ARMv7 architecture level both
for the check and for the actual compilation of the scm driver.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andy Gross <agross@codeaurora.org>