omap: rx51: Change the TWL4030 VMMC2 voltage constraints andsupply name

I believe the VMMC2 constraints must be the same than with VAUX3. Older
boards are using TWL4030 VMMC2 supply for internal MMC whereas newer are
using VAUX3 that has more limited constraints defined in this same file.

More over, the VMMC2 supply is used also for analog audio domain and the
miminum analog voltage of the TLV320AIC34 codec is 2.7 V.

To combine these two facts, the patch changes supply name to V28_A as the
newer boards register VMMC2_30 for VAUX3 and uses the same constraints than
VAUX3 since those constraints are ok for the TLV320AIC34.

Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Cc: Adrian Hunter <adrian.hunter@nokia.com>
Cc: Eduardo Valentin <eduardo.valentin@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>