hw/riscv/sifive_u.c: use intc_phandle in plic creation

The info about intc_phandles for each CPU is already stored in the
intc_phandles array.  We don't need to fetch them again using
qemu_fdt_get_phandle().

Signed-off-by: Daniel Henrique Barboza <daniel.barboza@oss.qualcomm.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20260616235939.1358663-3-daniel.barboza@oss.qualcomm.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>