imx:qspi add 4K erase support

Add 4k erase command support for qspi driver. reuse the 64k erase function,
but change the function name from qspi_op_se to qspi_op_erase, since it
supports 64k and 4k erase.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>