riscv: Add support to determine no. of L2 cache way enabled

In order to determine the number of L2 cache ways enabled at runtime,
implement a private attribute ("number_of_ways_enabled"). Reading this
attribute returns the number of enabled L2 cache ways at runtime.

Using riscv_set_cacheinfo_ops() hook a custom function, that returns
this private attribute, to the generic ops structure which is used by
cache_get_priv_group() in cacheinfo framework.

Signed-off-by: Yash Shah <yash.shah@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>