omap: Update and rename common defconfig for omap2plus devices

omap: Update and rename common defconfig for omap2plus devices

This patch addresses below things

- Renaming existing omap3_defconfig which has OMAP2, OMAP3 and OMAP4 builds
  enabled to more appropriate name 'omap2plus_defconfig'
- L1 cache shift is suppose to be 5 on OMAP4 where as it is 6 on previous
  OMAPs. Keeping it to 5 is safer option for OMAP4 and previous OMAPs. For
  OMAP3 only build the shift would be still 6
- Enable needed Errata's for OMAP4 to work with DMA based device drivers
	CONFIG_PL310_ERRATA_588369=y
	CONFIG_ARM_ERRATA_720789=y
- Enable the Micrel ethernet controller
	CONFIG_KS8851=y
	CONFIG_KS8851_MLL=y

With above three changes, OMAP4 ethernet, mmc etc works reliably.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>