dt-bindings: riscv: document cbom-block-size

The Zicbom operates on a block-size defined for the cpu-core,
which does not necessarily match other cache-sizes used.

So add the necessary property for the system to know the core's
block-size.

Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Guo Ren <guoren@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20220706231536.2041855-3-heiko@sntech.de
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>