riscv,interrupt: Allow checkIRQ to use full range

Any IRQs in the 0,maxIRQ range will be reserved by the kernel already if
userlevel is unable to access them.

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: Correct Arch_checkIRQ

This function now returns an error when the IRQ number is outside of the
acceptable range. Previously it always succeeded due to a bad
conditional.

Add RISC-V IRQ invocations

We add support for seL4_IRQControlGet and seL4_IRQControlGetTrigger. If
a platform doesn't support setting the trigger, then
seL4_IRQControlGetTrigger will return an error. If the platform doesn't
have a PLIC driver, then it won't have any IRQs that can be requested.

Also authored by: Siwei Zhuang <siwei.zhuang@data61.csiro.au>

style: consistently attach return type

Add attach-return-type to astyle

[SELFOUR-1156] RISC-V Port

Experimental release that supports both RV32 and RV64