riscv: Append ISA extensions to the device tree

The riscv,isa DT property only contains single letter base extensions
until now. However, there are also multi-letter extensions which were
ratified recently. Add a mechanism to append those extension details
to the device tree so that guest can leverage those.

Signed-off-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Link: https://lore.kernel.org/r/20220815101325.477694-3-apatel@ventanamicro.com


Signed-off-by: Will Deacon <will@kernel.org>