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.

secondary_data is written by the CPU that brings the secondary online with
the MMU enabled in the common case (that is, unless the user specifically
compiled the tests to run with the MMU disabled), and now it is read by the
secondary with the MMU disabled. Data is fetched from PoC by the secondary
when the MMU is disabled, clean struct secondary_data to PoC to handle
this.

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