davinci: handle CONFIG_SYS_CLE_MASK and CONFIG_SYS_ALE_MASK

these variables are curently defined in several config files but the
driver doesn't use them and defaults to hardcoded values in
nand_defs.h

It's interesting to be able to change this hardcoded valude when the
hardware is not using the default adress signals to drive ALE and CLE
and two configuration defines already exist for this purpose so use
them.

Signed-off-by: Eric Bénard <eric@eukrea.com>