arm: am437x: Enable hardware leveling for EMIF

Switch to using hardware leveling for certain parameters on the EMIF
rather than using precalculated values.  Doing this also means we have a
common place now between am437x and am335x for setting
emif_sdram_ref_ctrl with a value for the correct delay length.

Tested-by: Felipe Balbi <balbi@ti.com>
Tested-by: Tom Rini <trini@ti.com>
Signed-off-by: James Doublesin <doublesin@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>