lib: Move struct sbi_ipi_data definition to sbi_ipi.c

The struct sbi_ipi_data is only used in sbi_ipi.c so move it
to sbi_ipi.c from sbi_ipi.h.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>