dt-bindings: qcom: clk: Add missing binding for SDCHI enablement on Nexus 5X/6P

AHB clock branch is needed in order to enable SDHCI
on msm899(2/4).

Signed-off-by: Jeremy McNicoll <jeremymc@redhat.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>