ARM: omap2: mark unused functions as __maybe_unused

The omap_generic_init() and omap_hwmod_init_postsetup() functions are
used in the initialization for all OMAP2+ SoC types, but in the
extreme case that those are all disabled, we get a warning about
unused code:

arch/arm/mach-omap2/io.c:412:123: error: 'omap_hwmod_init_postsetup' defined but not used [-Werror=unused-function]
arch/arm/mach-omap2/board-generic.c:30:123: error: 'omap_generic_init' defined but not used [-Werror=unused-function]

This annotates both as __maybe_unused to shut up that warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>