arm: ls1021atwr: optimize the deep sleep latency

It will take more than 1s when wake up from deep sleep. Most of the
time is spent on outputing information. This patch reduced the deep
sleep latency by:
1. avoid outputing system informaton
2. remove flush cache after DDR restore
3. skip reloading second stage uboot binary when SD boot

Signed-off-by: Tang Yuantian <Yuantian.Tang@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>