target/riscv: fix invalid riscv,event-to-mhpmcounters entry

dt-validate complains:
> soc: pmu: {'riscv,event-to-mhpmcounters':
> [[1, 1, 524281], [2, 2, 524284], [65561, 65561, 524280],
> [65563, 65563, 524280], [65569, 65569, 524280], [0, 0, 0], [0, 0]],
> pmu: riscv,event-to-mhpmcounters:6: [0, 0] is too short

There are bogus 0 entries added at the end, of which one is of
insufficient length. This happens because only 15 of
fdt_event_ctr_map[]'s 20 elements are populated & qemu_fdt_setprop() is
called using the size of the array.
Reduce the array to 15 elements to make the error go away.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230404173333.35179-1-conor@kernel.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/pmu.c

@@ -35,7 +35,7 @@
  */
 void riscv_pmu_generate_fdt_node(void *fdt, int num_ctrs, char *pmu_name)
 {
-    uint32_t fdt_event_ctr_map[20] = {};
+    uint32_t fdt_event_ctr_map[15] = {};
     uint32_t cmask;
 
     /* All the programmable counters can map to any event */