ARM: drop unused Makefile.boot of Multiplatform SoCs

The variable "MACHINE" is empty if CONFIG_ARCH_MULTIPLATFORM=y,
so these Makefile.boot files are never included.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Michal Simek <michal.simek@xilinx.com> (for Zynq)
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Olof Johansson <olof@lixom.net>