lib: sbi: Simplify ipi platform operations

Instead of having ipi_send() and ipi_clear() callbacks in
platform operations, it will be much simpler for ipi driver
to directly register these operations as a device to sbi_ipi
implementation.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>