lib: Create a sbi_ipi_data structure

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>