riscv: add infrastructure for calling functions on other harts

Harts on RISC-V boot independently, U-Boot is responsible for managing
them. Functions are called on other harts with smp_call_function(),
which sends inter-processor interrupts (IPIs) to all other available
harts. Available harts are those marked as available in the device tree
and present in the available_harts mask stored in global data. The
available_harts mask is used to register all harts that have entered
U-Boot. Functions are specified with their address and two function
arguments (argument 2 and 3). The first function argument is always the
hart ID of the hart calling the function. On the other harts, the IPI
interrupt handler handle_ipi() must be called on software interrupts to
handle the request and call the specified function.

Functions are stored in the ipi_data data structure. Every hart has its
own data structure in global data. While this is not required at the
moment (all harts are expected to boot Linux), this does allow future
expansion, where other harts may be used for monitoring or other tasks.

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>
Tested-by: Bin Meng <bmeng.cn@gmail.com>