ARM: OMAP2+: Fix omap24xx_io_desc warning if SoC subtypes are not selected

Otherwise we get the following warning:

arch/arm/mach-omap2/io.c:53:24: warning:
'omap24xx_io_desc' defined but not used [-Wunused-variable]

Reported-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>