powerpc: select PPC_CLOCK unconditionally for FSL_SOC

Freescale PowerPC SoCs share a number of IP blocks with Freescale
ARM/IMX SoCs, FlexCAN, SSI, FEC, eSDHC, USB, etc.  There are some
effort consolidating those drivers to make them work for both
architectures.

One outstanding difference between two architectures is ARM/IMX will
turn off module clocks during platform initialization for power saving
and expects drivers manage clocks using clk API, while PowerPC
mostly does not do that, and thus does not always build in clk API.

Listing all those driver Kconfig options in "select PPC_CLOCK if" seems
not scalable for long term maintenance, and could easily introduce
Kconfig recursive dependency.  This patch chooses to select PPC_CLOCK
unconditionally for FSL_SOC to always build clk API for PowerPC in.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>