arm64: Use generic Layerscape SoC family naming

Freescale will be a spinning-out a set of ARMv8 based SoCs which
will be based on a similar overall SoC architecture. So, this patch
converts the existing infrastructure in the arm64/dts, arm64/Kconfig
and arm64/configs to use the generic convention ARCH_LAYERSCAPE
in place of the more specific FSL_LS2085A, to save code duplication
later-on.

Signed-off-by: Bhupesh Sharma <bhupesh.sharma@freescale.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>