ASoC: omap: enable sound support on n900 on devicetree-based boot

With device tree, it is possible (and encouraged) to build N900
kernels without CONFIG_MACH_NOKIA_RX51. Update config file to enable
the driver build in this case.

This makes sound work on my n900 under 3.18-rc1.

Signed-off-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Mark Brown <broonie@kernel.org>