riscv: Add Zicbom extension support

When the Zicbom extension is available expose it to the guest.
Also provide the guest the size of the cache block through DT.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20221018140854.69846-6-apatel@ventanamicro.com


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