of: base: add support to find the level of the last cache

It is useful to have helper function just to get the number of cache
levels for a given logical cpu. We can obtain the same by just checking
the level at which the last cache is present. This patch adds support
to find the level of the last cache for a given cpu.

It will be used on ARM64 platform where the device tree provides the
information for the additional non-architected/transparent/external
last level caches that are not integrated with the processors.

Cc: Mark Rutland <mark.rutland@arm.com>
Suggested-by: Rob Herring <robh+dt@kernel.org>
Acked-by: Rob Herring <robh+dt@kernel.org>
Tested-by: Tan Xiaojun <tanxiaojun@huawei.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
[will: use u32 instead of int for cache_level]
Signed-off-by: Will Deacon <will.deacon@arm.com>