hwspinlock: Add one new API to support getting a specific hwlock by the name

The hardware spinlock binding already supplied the 'hwlock-names' property
to match and get a specific hwlock, but did not supply one API for users
to get a specific hwlock by the hwlock name. So this patch introduces one
API to support this requirement.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>