sf: Read flash bank addr register at probe time

Read the flash bank addr register to get the state of bank in
a perticular flash. and also bank write happens only when there is
a change in bank selection from user.

bank read only valid for flashes which has > 16Mbytes those are
opearted in 3-byte addr mode, each bank occupies 16Mytes.

Suppose if the flash has 64Mbytes size consists of 4 banks like
bank0, bank1, bank2 and bank3.

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>