Merge branch 'arm-multiplatform-cleanup' of https://git.kernel.org/pub/scm/linux/kernel/git/soc/soc into arm/soc

Now that everything except StrongARM is unified under
CONFIG_ARCH_MULTIPLATFORM, the option is rather meaningless
in its current form.

Rework the Kconfig logic to make this useful again, similar
to the way that RISC-V has CONFIG_NONPORTABLE (with the
opposite polarity), this now controls the visibility of
options that get in the way of building generic kernels,
while allowing custom kernels.

One side-effect is that 'randconfig' builds now rarely hit
strongarm machines, rather than testing them three quarters
of the time.

* 'arm-multiplatform-cleanup' of https://git.kernel.org/pub/scm/linux/kernel/git/soc/soc:
  ARM: make ARCH_MULTIPLATFORM user-visible
  ARM: fix XIP_KERNEL dependencies
  ARM: Kconfig: clean up platform selection
  ARM: simplify machdirs/platdirs handling
  ARM: remove obsolete Makefile.boot infrastructure