arm: mvebu: axp: Set CONFIG_SYS_TCLK globally

This mvebu axp platform always uses fixed 250 MHz TCLK. So specify this
CONFIG_SYS_TCLK option in msys section of global file soc.h file instead of
manual configuration in every board file.

Now every #if-#else case of soc.h file defines CONFIG_SYS_TCLK, so remove
useless default CONFIG_SYS_TCLK value from the end of soc.h file.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>