riscv: implement IPI platform functions using SBI

The supervisor binary interface (SBI) provides the necessary functions
to implement the platform IPI functions riscv_send_ipi() and
riscv_clear_ipi(). Use it to implement them.

This adds support for inter-processor interrupts (IPIs) on RISC-V CPUs
running in supervisor mode. Support for machine mode is already
available for CPUs that include the SiFive CLINT.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>