[riscv] Add support for checking CPU extensions reported via device tree

[riscv] Add support for checking CPU extensions reported via device tree

RISC-V seems to allow for direct discovery of CPU features only from
M-mode (e.g. by setting up a trap handler and then attempting to
access a CSR), with S-mode code expected to read the resulting
constructed ISA description from the device tree.

Add the ability to check for the presence of named extensions listed
in the "riscv,isa" property of the device tree node corresponding to
the boot hart.

Signed-off-by: Michael Brown <mcb30@ipxe.org>