ar71xx: fix nand init issues on some rb2011 devices

ar71xx: fix nand init issues on some rb2011 devices

While flashing lots of RB2011 devices, I noticed that
some of them refused to boot properly, failing over the NAND parameters.

Checking in detail shows that some device seem to use another NAND flash
which only support standard 2048-byte pages, without 512-byte subpage support.

This commit disables usage of these small subpage completely.

Advantages:
- Both NAND's with(out) subpage support are working now
- The nand speed increases a bit (measured roughly 1%) in typical usecases

Disadvantages:
- The maximum storage capacity decreases by ~0.2%
as small changes can consume a full page (2048 bytes) now.

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>