irqchip/irq-xtensa-pic: Clean up

  - get rid of the cached_irq_mask variable
  - use BIT() macro instead of bit shifts
  - drop .disable and .enable as they are equivalent to the default
    implementations

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20231208163857.82644-1-jcmvbkbc@gmail.com