Kbuild: introduce Makefile in arch/$ARCH/

Introduce a Makefile under arch/$ARCH/ and include it in the
top Makefile (similar to Linux kernel). This allows further
refactoringi like moving architecture-specific code out of global
makefiles, deprecating config variables (CPU, CPUDIR, SOC) or
deprecating arch/$ARCH/config.mk.

In contrary to Linux kernel, U-Boot defines the ARCH variable by
Kconfig, thus the arch Makefile can only included conditionally
after the top config.mk.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Acked-by: Masahiro Yamada <yamada.m@jp.panasonic.com>