doc: restore doc/README.fsl-clk

doc: restore doc/README.fsl-clk

doc/README.fsl-clk was removed in commit
5b8e76c35ec31 ("powerpc, 8xx: remove support for 8xx")
allthought CONFIG_SYS_FSL_CLK is defined in
arch/arm/cpu/armv8/fsl-layerscape/Kconfig and still in use
in the following configs:

./include/configs/mx53loco.h:21:#define CONFIG_SYS_FSL_CLK
./include/configs/m53evk.h:16:#define CONFIG_SYS_FSL_CLK
./include/configs/mx25pdk.h:17:#define CONFIG_SYS_FSL_CLK
./include/configs/usbarmory.h:14:#define CONFIG_SYS_FSL_CLK
./include/configs/ls1021aqds.h:14:#define CONFIG_SYS_FSL_CLK
./include/configs/mx53cx9020.h:22:#define CONFIG_SYS_FSL_CLK
./include/configs/colibri_vf.h:17:#define CONFIG_SYS_FSL_CLK
./include/configs/mx35pdk.h:21:#define CONFIG_SYS_FSL_CLK
./include/configs/woodburn_common.h:19:#define CONFIG_SYS_FSL_CLK
./include/configs/mx7_common.h:25:#define CONFIG_SYS_FSL_CLK
./include/configs/ls1021aiot.h:12:#define CONFIG_SYS_FSL_CLK
./include/configs/ls1021atwr.h:14:#define CONFIG_SYS_FSL_CLK
./include/configs/mx53ard.h:21:#define CONFIG_SYS_FSL_CLK
./include/configs/mx53smd.h:21:#define CONFIG_SYS_FSL_CLK
./include/configs/mx51evk.h:16:#define CONFIG_SYS_FSL_CLK
./include/configs/mx6_common.h:31:#define CONFIG_SYS_FSL_CLK
./include/configs/vf610twr.h:14:#define CONFIG_SYS_FSL_CLK
./include/configs/mx53evk.h:21:#define CONFIG_SYS_FSL_CLK

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Heiko Schocher <hs@denx.de>