xtensa: fix CONFIG_NUM_IRQS if !CONFIG_*_LEVEL_INTERRUPTS

CONFIG_NUM_IRQS blindly assumes CONFIG_2ND_LEVEL_INTERRUPTS
and CONFIG_3RD_LEVEL_INTERRUPTS are always enabled together,
which is not always the case. So fix the #define.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>