ARM: omap1: fix build with no SoC selected

In a multiplatform randconfig kernel, one can have
CONFIG_ARCH_OMAP1 enabled, but none of the specific SoCs.
This leads to some build issues as the code is not
meant to deal with this configuration at the moment:

arch/arm/mach-omap1/io.c:86:20: error: unused function 'omap1_map_common_io' [-Werror,-Wunused-function]
arch/arm/mach-omap1/pm.h:113:2: error: "Power management for this processor not implemented yet" [-Werror,-W#warnings]

Use the same trick as on OMAP2 and guard the actual compilation
of platform code with another Makefile ifdef check based
on an option that depends on having at least one SoC enabled.

The io.c file still needs to get compiled to allow building
device drivers with a dependency on CONFIG_ARCH_OMAP1.

Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>