softpll-unify: rename gpio.h to gpio-wrs.h (no effect on rt_cpu.bin)

The commit is part of the effort in unifying softpll with wrpc-sw,
and later remove the duplicated code here in wr-switch-sw.

While we have no gpio.h in wrpc-sw, it's better to clarify this
header is for wr-switch.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>

rt/dev/ad9516.c

@@ -9,7 +9,7 @@
 
 #include "board.h"
 #include "syscon.h"
-#include "gpio.h"
+#include "gpio-wrs.h"
 
 #include "rt_ipc.h"