Finish converting CONFIG_SYS_FSL_CLK to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_FSL_CLK

We move the exiting option to common/Kconfig near the other options to
control the contents of board_init_f() and note that this is a legacy
option.  We further restrict this to where the call is going to be
non-empty, for the SoCs that had only been using this for some
MMC-related clocks.

Signed-off-by: Tom Rini <trini@konsulko.com>