mtd: spi-nor: Changes for stacked and parallel

This patch adds support for dual parallel and stacked configurations.

The existing "is_dual" property and DUAL_STACK config options were used
to identify the hw configurations.
Added required parameters to spi_nor structure and initialized the same
in m25p80 probe or spi_nor_scan.
Added support for dual stacked and parallel in spi_nor_read/write/erase
functions by doing the following:
- Increase page_size, sector_size, erase_size and total flash size as
  applicable.
- Halve address when in parallel mode.
- When in stacked subtract lower flash address from offset when addressing
  upper chip. Update spi->master->flags so that qspi driver can change CS.

spansion_quad_enable/set_quad_mode are still left unused.

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