msm: kgsl: Declare platform_bus_type as secure for ARM driver

ARM driver supports only one bus for all context banks.

In some cases, hypervisor may not be available and GPU SMMU
uses ARM driver. This will make all context banks are on
non secure bus and kgsl_mmu_bus_secured() returns -EPERM.

Make platform_bus_type as secure for ARM driver.

Change-Id: I11a637ca2b1ef29cc42c9811cad009312a2879cd
Signed-off-by: Rajesh Kemisetti <rajeshk@codeaurora.org>