riscv: Switch the FPU owner when migrating a TCB

If a TCB is being migrated to another core and it is the current
FPU owner, we reset the FPU owner to NULL.

Signed-off-by: Yanyan Shen <yanyan.shen@data61.csiro.au>

Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

riscv: Add Arch_migrateTCB function for SMP

Add an empty function to pass compilation at the moment.

Remove platform IPC buffer register.

This removes the assumption that each platform sotres the IPC buffer
address in a platform-specific register. The IPC buffer address is
instead stored in a thread-local variable in libsel4 which must be
initialised by the runtime.

style: consistently attach return type

Add attach-return-type to astyle

riscv: Remove currently unuspported SMP code

riscv: Remove currently unsupported FPU

riscv: remove HARTID

RISCV: Place TODOs in the source

[SELFOUR-1156] RISC-V Port

Experimental release that supports both RV32 and RV64