ARM: OMAP3: Add back Kconfig option MACH_OMAP3517EVM for ASoC

We still have SND_OMAP_SOC_AM3517EVM depending on MACH_OMAP3517EVM,
so let's keep MACH_OMAP3517EVM Kconfig option around for a little
bit longer.

This removes the dependency between ARM SoC changes and the ASoC
changes, and allows the following three options for the driver:

1. Update the driver for device tree based booting

2. Initialize the driver with legacy platform data, then update
   the driver for device tree based booting

3. Just remove the driver if there are no audio users for
   3517-evm board

Reported-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Tony Lindgren <tony@atomide.com>