nand_spl: nand_boot.c: Remove CONFIG_SYS_NAND_READ_DELAY

There are multiple reasons why this define should be removed:

First it saves some space and therefore fixes a problem we have on
the canyonlands_nand and glacier_nand targets right now.

Second, the define was hackish and would most likely not work on all
board using nand_boot.c. Boards not providing a real dev_ready()
function should implement a board specific function instead.

I checked and it seems, that all boards using nand_boot.c right now
already implement a board specific dev_ready() function. So this
patch should not break any boards and will result in smaller
NAND_SPL images.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Sughosh Ganu <urwithsughosh@gmail.com>
Cc: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
Tested-by: Sughosh Ganu <urwithsughosh@gmail.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>