spi: imx: Define register bits in the driver

The CSPI/ECSPI register bits do not differ between newer SoCs, instead
of having multiple copies of the same thing for each iMX SoC, define
the bits in the driver.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Stefano Babic <sbabic@denx.de>