ARM: dts: msm: Remove unneeded context bank in GPU dt

The SMMU only has 2 SMRs for the GPU. Therefore, when we try to
attach a 3rd time for the secure context bank, it fails.

Ideally, the SMMU driver would have a masking option so that they
could use one SMR for secure and the other for non-secure.
However, we will go with this method since removing this from the
DT doesn't have a consequence for us.

Change-Id: Ib0934614976e94d0a1ceb166ec8155e4e9e78c40
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>