ARM: imx: Update spi_imx platform data to reflect current state

The docs for the spi_imx platform data still refer to a -32 offset used to
specify a native chip select.  This was removed in commit 602c8f44
("spi: imx: fix use of native chip-selects with devicetree") and no
longer works as documented.  Update documentation.

The macro MXC_SPI_CS() is no longer is needed.

If a board uses all native chip selects, then it's not necessary to
specify a chip select array at all, as all native is the default (this is
how device-tree configured SPI masters work too).  Most of the spi-imx
platform data users have their chip select arrays removed by this patch.

This patch also fixes a bug in mx31moboard introduced in the '602 commit.
When that board was updated in commit 901f26bc ("ARM: imx: set
correct chip_select in platform setup") to reflect the SPI change, only
SPI bus 2 was updated and SPI bus 1 was left with non-sequential chip
selects.  The mc13783 spi device on bus 1 had its chip select updated as
if it were on bus 2.

CC: Sascha Hauer <kernel@pengutronix.de>
CC: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>