Get rid of bogus CONFIG_SYS_BUS_HZ and CONFIG_SYS_CONFIG_BUS_CLK definitions

CONFIG_SYS_BUS_HZ has not really been used anywhere except to be
redined as CONFIG_SYS_BUS_CLK; in addition, the mpc7448hpc2 had the
bogus CONFIG_SYS_CONFIG_BUS_CLK setting which duplicated the
funtionality.  Change all this to use CONFIG_SYS_BUS_CLK consistently.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Frank Gottschling <fgottschling@eltec.de>
Cc: Reinhard Arlt <reinhard.arlt@esd-electronics.com>
Cc: Eran Man <eran@nbase.co.il>
Cc: Stefan Roese <sr@denx.de>
Cc: Nye Liu <nyet@zumanetworks.com>
Cc: Roy Zang <tie-fei.zang@freescale.com>