firmware: Not to clear all the MIP

In generic behavior of QEMU, if the pending bits of PLIC are still set and
we clear the SEIP, the QEMU may not set the SEIP back immediately and the
interrupt may not be handled anymore until the new interrupts arrived and
QEMU set the SEIP back which is a generic behavior in QEMU.

Signed-off-by: Nick Hu <nick.hu@sifive.com>
Signed-off-by: Jim Shu <jim.shu@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>