pfla02: Fix RAM detection and support 1 bank SOM

In case of 2 banks, the address space of the first CS must be defined
and not let to the higher value.

Add support for SOM with a single bank of RAM. It was tested with i.MX6Q
modules in the following configurations:

- 2 Banks, 4 GB
- 2 Banks, 1 GB
- 1 Bank,  1 GB

Signed-off-by: Stefano Babic <sbabic@denx.de>