ARM: pxa: reduce the scope of get_clk_frequency_khz()

get_clk_frequency_khz() is used in private cpufreq driver. In order to meet
the change of different pxa silicons, checking cpuid is introduced in
get_clk_frequency_khz(). While more pxa silicons are supported, the workload
of checking cpuid is higher.

So restrict the scope of get_clk_frequency_khz() on pxa2xx. Different pxa
silcions use different private cpufreq driver to avoid too much checking
on cpuid.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Cc: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>