Don't call printk() when CONFIG_PRINTK is not set.
Fixes the following build errors:

or1k-linux-ld: arch/openrisc/kernel/entry.o: in function `_external_irq_handler':
(.text+0x804): undefined reference to `printk'
(.text+0x804): relocation truncated to fit: R_OR1K_INSN_REL_26 against undefined symbol `printk'

Fixes: 9d02a428 ("OpenRISC: Boot code")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Cc: Stafford Horne <shorne@gmail.com>
Cc: openrisc@lists.librecores.org
Signed-off-by: Stafford Horne <shorne@gmail.com>