mx31: Define default SoC input clock frequencies

Define default SoC input clock frequencies for i.MX31 in order to get rid of
duplicated definitions.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Helmut Raiger <helmut.raiger@hale.at>