arm: Don't try to support CONFIG_ARMV7_LPAE on ARMv4T

At present if CONFIG_ARMV7_LPAE is defined then mmu_setup() will use
instructions which are invalid on ARMv4T. This happens on Tegra since it
has an ARMv4T boot CPU. Add a check for the architecture version to allow
the code to be built. It will not actually be executed by the boot CPU,
but needs to compile.

Signed-off-by: Simon Glass <sjg@chromium.org>