Create a sbi_ipi_data structure that holds unpacked IPI information. At
the same time remove ipi_type from the sbi_scratch struct and use a
fixed offset to access it.
This structure fits in behind the sbi_scratch structure.
This fixes https://github.com/riscv/opensbi/issues/81
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>