iommu/qcom: add optional 'tbu' clock for TLB invalidate

On some SoCs like MSM8939 with A405 adreno, there is a gfx_tbu clock
needs to be on while doing TLB invalidate. Otherwise, TLBSYNC status
will not be correctly reflected, causing the system to go into a bad
state.  Add it as an optional clock, so that platforms that have this
clock can pass it over DT.

While adding the third clock, let's switch to bulk clk API to simplify
the enable/disable calls.  clk_bulk_get() cannot used because the
existing two clocks are required while the new one is optional.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20200518141656.26284-1-shawn.guo@linaro.org
Signed-off-by: Joerg Roedel <jroedel@suse.de>