Remove the custom low-level initialization function and reuse the
default low-level initialization function. But this requires the
ARMV8_MULTIENTRY config option to be enabled for Exynos7420.

On Exynos7420, the boot CPU belongs to the second cluster and so
with ARMV8_MULTIENTRY config option enabled, the 'branch_if_master'
macro fails to detect the CPU as boot CPU. As a temporary workaround
the CPU_RELEASE_ADDR is set to point to '_main'.

Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Alison Wang <alison.wang@nxp.com>
Signed-off-by: Thomas Abraham <thomas.ab@samsung.com>
Reviewed-by: Alison Wang <alison.wang@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>