powerpc: Split systemcfg struct definitions out from vdso

The systemcfg data has nothing to do anymore with the vdso.
Split it into a dedicated header file.

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-v1-27-b64f0842d512@linutronix.de