powerpc: Revert show_regs() define for readability

We had an existing ifdef for 4xx & BOOKE processors that got changed to
CONFIG_PPC_ADV_DEBUG_REGS.  The define has nothing to do with
CONFIG_PPC_ADV_DEBUG_REGS.  The define really should be:

 #if defined(CONFIG_4xx) || defined(CONFIG_BOOKE)

and not

 #ifdef CONFIG_PPC_ADV_DEBUG_REGS

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>