RISC-V: Use atomic_cmpxchg to update PLIC bitmaps

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>

include/hw/riscv/sifive_plic.h

@@ -55,7 +55,6 @@ typedef struct SiFivePLICState {
     uint32_t *pending;
     uint32_t *claimed;
     uint32_t *enable;
-    QemuMutex lock;
 
     /* config */
     char *hart_config;