[ARM] pxa: cpufreq-pxa2xx: sdram_rows detection support

This patch implements Eric Miao's idea to detect the correct value of
sdram_rows by inspecting the MDCNFG register settings.
It is only tested on two pxa27x devices with 64MB RAM (magician and
hx4700) so far.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Eric Miao <eric.miao@marvell.com>