riscv: andes_plic: Fix riscv_get_ipi() mask

Current logic in riscv_get_ipi() for Andes PLICSW does not look
correct. The mask to test IPI pending bits for a hart should be
left shifted by (8 * gd->arch.boot_hart), just the same as what
is done in riscv_send_ipi().

Fixes: 8b3e97ba ("riscv: add functions for reading the IPI status")
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Tested-by: Rick Chen <rick@andestech.com>