lib: sbi: Allow relaxed MMIO writes in device ipi_send() callback

Currently, we have a smp_wmb() between atomic_raw_set_bit() and
ipi_send() device callback whereas the MMIO writes done by the
device ipi_send() callback will also include a barrier.

We can avoid unnecessary/redundant barriers described above by
allowing relaxed MMIO writes in device ipi_send() callback. To
achieve this, we simply use  wmb() instead of smp_wmb() before
calling device ipi_send().

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reported-by: Bo Gan <ganboing@gmail.com>