TI EDMA: Enable Kconfig options for Davinci

* Share CONFIG option for TI_EDMA between TI_PRIV_EDMA, since they
are coupled, build both of them with single config option- TI_EDMA

* Select DMADEVICES for building drivers/dma for Davinci which is
required for building TI EDMA driver.

* Further, build TI_EDMA by default if DMADEVICES is selected by
DAVINCI or OMAP ARCHs. One might argue that EDMA is being built by
default for all mach-omap2plus, but it is require for atleast some
platforms with EDMA to boot (mount rootfs) so we enable it.

* Add MMC and EXT4 options to da8xx_omapl_defconfig

The idea is to give one an out-of-the-box experience when building
omap2plus_defconfig or other Davinci defconfig.

Signed-off-by: Joel A Fernandes <joelagnel@ti.com>