OMAP2+: sdrc: fix compile break on OMAP4-only config on current omap-for-linus

On non-OMAP2 and non-OMAP3 kernel configs, turn omap2_sdrc_init() into
a no-op.  Otherwise, compilation breaks on an OMAP4-only config with
the current omap-for-linus branch:

arch/arm/mach-omap2/built-in.o: In function `omap2_init_common_devices':
../mach-omap2/io.c:421: undefined reference to `omap2_sdrc_init'

Thanks to Sergei Shtylyov <sshtylyov@mvista.com> for suggesting the use
of a empty static inline function rather than a macro.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Sergei Shtylyov <sshtylyov@mvista.com>
[tony@atomide.com: updated not to use __init for inline omap2_sdrc_init]
Signed-off-by: Tony Lindgren <tony@atomide.com>