p2020ds: add alternate boot bank support using the ngPIXIS FPGA

The Freescale P2020DS board uses a new type of PIXIS FPGA, called the ngPIXIS.
The ngPIXIS has one distinct new feature: the values of the on-board switches
can be selectively overridden with shadow registers.  This feature is used to
boot from a different NOR flash bank, instead of having a register dedicated
for this purpose.  Because the ngPIXIS is so different from the previous PIXIS,
a new file is introduced: ngpixis.c.

Also update the P2020DS checkboard() function to use the new macros defined
in the header file.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>