[U-Boot] [PATCH] Add SPI Flash STMicro's N25Q512A & add flag status check during SPI Flash write

[Insop Song]

Hi,

I've made two changes while I was working on STMidro's SPI Flash N25Q512A
- add the device entry for STMidro's SPI Flash N25Q512A
- add Flag status check during 
	Without this flag checking "sf write" will fail and SPI flash is locked up

Please see the patch and let me know your feedback.
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