powerpc/sgy_cts1000: Fix gpio_halt_cb()'s signature

Halt callback in struct machdep_calls is declared with __noreturn
attribute, so omitting that attribute in gpio_halt_cb()'s signatrue
results in compilation error.

Change the signature to address the problem as well as change the code
of the function to avoid ever returning from the function.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Scott Wood <oss@buserror.net>