The PLIC previously used a mutex to protect against concurrent
access to the claimed and pending bitfields. Instead of using
a mutex, we update the bitfields using atomic_cmpxchg.
Rename sifive_plic_num_irqs_pending to sifive_plic_irqs_pending
and add an early out if any interrupts are pending as the
count of pending interrupts is not used.
Cc: Sagar Karandikar <sagark@eecs.berkeley.edu> Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de> Cc: Palmer Dabbelt <palmer@sifive.com> Cc: Alistair Francis <Alistair.Francis@wdc.com> Signed-off-by: Michael Clark <mjc@sifive.com> Reviewed-by: Richard Henderson <richard.henderson@linaro.org> Signed-off-by: Alistair Francis <alistair.francis@wdc.com>