sunxi: Use udelay() in dramc_init rather than sdelay()

Use udelay() in raminit code, as the delays provided by sdelay depend
on a number of variables, including the CPU clock. udelay() is already
implemented via timer 0.
We measured about 8 loops/tick (timer @ 24MHz), or about about 192
sdelay loops per microsecond. Hence why the arguments to udelay are
192 times lower than the arguments to sdelay to provide the same
delay.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>