fec_mxc: cleanup and factor out MX27 dependencies

general cleanup
move clock init to cpu_eth_init in cpu/arm926ejs/mx27/generic.c
make MX27 specific phy init conditional on CONFIG_MX27
replace call to imx_get_ahbclk with one to imx_get_fecclk
and define imx_get_fecclk in include/asm-arm/arch-mx27/clock.h

Signed-off-by: John Rigby <jcrigby@gmail.com>
CC: Ben Warren <biggerbadderben@gmail.com>
CC: Fred Fan <fanyefeng@gmail.com>
CC: Tom <Tom.Rix@windriver.com>