Documentation: dt: Update ti,emif bindings

Update the Texas Instruments EMIF binding document to include the device
tree bindings for ti,emif-am3352 and ti,emif-am4372 which are used by
the ti-emif-sram driver to provide low-level PM functionality.

Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>