powerpc: Convert print_symbol to %pSR

Use the new vsprintf extension to avoid any possible
message interleaving.

Convert the #ifdef DEBUG block to a single pr_debug.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>