ARM: PSCI: Remove unused psci_text_end symbol

psci_text_end was used to calculate the PSCI stack address following the
secure monitor text. Now that we have an explicit secure stack section,
this is no longer used.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>