Tegra: smmu: make the context save sequence robust

This patch sanity checks the SMMU context created by the platform
code. The first entry contains the size of the array; which the
driver now verifies before moving on with the save.

This patch also fixes an error in the calculation of the size of
the context that gets copied to TZDRAM.

Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>