zynqmp: Remove dependency on arm_helpers.S

Non-Arm platforms shouldn't use Arm platform code. This patch copies the
implementation of the functions in arm_helpers.S to zynqmp_helpers.S to
remove this dependency of zynqmp on Arm platforms.

Change-Id: Ia85f303c4c63bcf0ffa57c7f3ef9d88376729b6b
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>