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

Currently, there are no barriers before or after the ipi_clear()
device callback which forces ipi_clear() device callback to always
use non-relaxed MMIO writes.

Instead of above, we use wmb() in after the ipi_clear() device
callback which pairs with the wmb() done before the ipi_send()
device callback. This also allows device ipi_clear() callback
to use relaxed MMIO writes.

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