cfi flash: add status polling method for amd flash

This patch adds status polling method to offer an alternative to
data toggle method for amd flash chips.

This patch is needed for nios2 cfi flash interface, where the bus
controller performs 4 bytes read cycles for a single byte read
instruction. The data toggle method can not detect chip busy
status correctly. So we have to poll DQ7, which will be inverted
when the chip is busy.

This feature is enabled with the config def,
CONFIG_SYS_CFI_FLASH_STATUS_POLL

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Stefan Roese <sr@denx.de>