arm/arm64: Configure secondaries' stack before enabling the MMU

Now that the secondaries' stack is linearly mapped, we can set it before
turning the MMU on. This makes the entry code for the secondaries
consistent with the entry code for the boot CPU.

To keep it simple, the struct secondary_data, which is now read with the
MMU off by the secondaries, is cleaned to PoC.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>