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