davinci: add support for aemif timing configuration

This patch adds support to configure the AEMIF interface
with supplied timing values.

Since this capability is useful both from NOR and NAND
flashes, it is provided as a new interface and in a file
of its own.

AEMIF timing configuration is required in cases:

1) Where the AEMIF clock rate can change at runtime (a side
   affect of cpu frequency change).

2) Where U-Boot does not support NAND/NOR but supports other
   media like SPI Flash or MMC/SD and thus does not care about
   setting up the AEMIF timing for kernel to use.

3) Where U-Boot just hasn't configured the timing values and
   cannot be upgraded because the box is already in the field.

Since there is now a header file for AEMIF interface, the
common (non-NAND specific) defines for AEMIF registers have
been moved from nand.h into the newly created aemif.h

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Acked-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>