arm: mx6: cm_fx6: detect 1GB DRAM correctly on solo

The 1GB DRAM configuration on mx6 solo uses 2 chip selects, but
the code tests 1GB DRAM configuration as if it is all present on one
chip select, and thus cannot see the full range of available memory.

Refactor the check to detect 1GB DRAM correctly.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Stefano Babic <sbabic@denx.de>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>