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>
sunxi/timer: Prevent overflow condition in udelay

The timer ticks were calculated by first multiplying the usec
parameter by 24000, then dividing by 1000. This would cause an
overflow on delays greater than about 178 milliseconds. Instead, use
the USEC_TO_COUNT() macro, which overflows only if the timer rolls
over.

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