GEODE: use symbolic constant in cs5536 reboot fixup

Simple cosmetic update for the cs5536 reboot fixup; define the MSR that's used
for rebooting in geode.h, and use the define.

Signed-off-by: Andres Salomon <dilinger@debian.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>