ARM: pxa: reduce the scope of get_memclk_frequency_10khz()

Up to now, only pxa2xx pcmcia driver is using the API. No other device driver
is using this API in PXA3xx or any other PXA silicons.

Restrict the scope only on pxa2xx and remove the implementation of pxa3xx. So
we can avoid oo much checking on cpuid after more pxa chips supported.

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>