powerpc: Add kconfig option for the systemcfg page

The systemcfg page through procfs is only a backwards-compatible
interface for very old applications.
Make it possible to be disabled.

This also creates a convenient config #define to guard any accesses to
the systemcfg page.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20241010-vdso-generic-base-v1-25-b64f0842d512@linutronix.de

arch/powerpc/kernel/proc_powerpc.c

@@ -14,7 +14,7 @@
 #include <asm/rtas.h>
 #include <linux/uaccess.h>
 
-#ifdef CONFIG_PPC64
+#ifdef CONFIG_PPC64_PROC_SYSTEMCFG
 
 static loff_t page_map_seek(struct file *file, loff_t off, int whence)
 {
@@ -59,7 +59,7 @@ static int __init proc_ppc64_init(void)
 }
 __initcall(proc_ppc64_init);
 
-#endif /* CONFIG_PPC64 */
+#endif /* CONFIG_PPC64_PROC_SYSTEMCFG */
 
 /*
  * Create the ppc64 and ppc64/rtas directories early. This allows us to