mtd: spi-nor: Support for > 128 Mbit flash devices

This patch adds support for >128Mbit flash devices.
While the default in spi-nor.c is 4 byte addressing, this patch adds
bank selection and uses the same 3 byte read commands.
Required parameters are added in spi_nor struct.
read_ear and write_ear functions are added with support for commands
used by spansion and micron.
spi_nor_write_ext and spi_nor_read_ext will:
- Check if bank change is required for current address to be accessed and
  change the bank.
- Call spi_nor_read/spi_nor_write functions for an operations within the
  bank bounds.
- Repeat above till required transfer is completed.

Signed-off-by: Harini Katakam <harinik@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>