omap4: automatic sdram detection

Identify SDRAM devices connected to EMIF automatically:
LPDDR2 devices have some Mode Registers that provide details
about the device such as the type, density, bus width
etc. EMIF has the capability to read these registers. If there
are no devices connected to a given chip-select reading mode
registers will return junk values. After reading as many such
registers as possible and matching with expected ranges of
values the driver can identify if there is a device connected
to the respective CS. If we identify that a device is connected
the values read give us complete details about the device.

This along with the base AC timings specified by JESD209-2
allows us to do a complete automatic initialization of
SDRAM that works on all boards.

Please note that the default AC timings specified by JESD209-2
will be safe for all devices but not necessarily optimal. However,
for the Elpida devices used on Panda and SDP the default timings
are both safe and optimal.

Signed-off-by: Aneesh V <aneesh@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>