GCC9 now produces the following warning:

common.h:51:2: warning: listing the stack pointer register ‘sp’ in a clobber list is deprecated [-Wdeprecated]
   51 |  __asm__ __volatile__("mov sp, %0"
      |  ^~~~~~~
common.h:51:2: note: the value of the stack pointer after  an ‘asm’ statement must be the same as it was before the statement

Stack pointer was added to clobber list in commit f9fc8254b2 ("ARM:
Mark SP as being clobbered in arm_setup_stack()") to prevent GCC from
generating code that would corrupt 'boarddata' pointer by trying to
restore it from invalid stack frame.

Interestingly enough, seemingly unrelated change in commit
64d95896cf ("ARM: aarch64: compile with general-regs-only") changed
generated code such that adding SP to clobber list became no longer
necessary.

While the above can probably be a fix by itself, it seems a better and
more future proof approach would be to address the problem at its root
and re-implement offending startup sequence in assembly.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>