nand_spl: update udelay for Freescale boards

nand_spl: update udelay for Freescale boards

Let's use the more appropriate udelay for the nand_spl. While we
can't make use of u-boot's full udelay we can atl east use a for
loop that won't get optimized away .Since we have the bus clock
we can use the timebase to calculate wall time.

Looked at reusing the u-boot udelay functions but it pulls in a lot
of code and would require quite a bit of work to keep us within the
very small space constrains we currently have

Signed-off-by: Matthew McClintock <msm@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>