clk: qcom: clk-rpmh: Mark rfclka2 as optional for Lahaina

Mark rfclka2 as an optional clock for Lahaina. This clock is currently
unused on our internal development boards and thus isn't populated in
cmd_db. It could be used on future boards, so don't remove it entirely.

Change-Id: I0f216e14e675196d4817c89db730d40f1e20aba1
Signed-off-by: Mike Tipton <mdtipton@codeaurora.org>