kbuild: prepare for moving headers into mach-*/include/mach

In U-Boot, SoC-specific headers are placed in
arch/$(ARCH)/include/asm/arch-$(SOC) and a symbolic link to that
directory is created at the early stage of the build process.

Creating and removing a symbolic link during the build is not
preferred.  In fact, Linux Kernel did away with include/asm-$(ARCH)
directories a long time time ago.

As for ARM, now it is possible to collect SoC sources into
arch/arm/mach-$(SOC).  It is also reasonable to move SoC headers
into arch/arm/mach-$(SOC)/include/mach.

This commit prepares for that.
If the directory arch/$(ARCH)/mach-$(SOC)/include/mach exists,
a symbolic to that directory is created.  Otherwise, a symbolic link
to arch/$(ARCH)/include/asm/arch-$(SOC) or arch-$(CPU) is created.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>