mpc83xx: Get rid of CONFIG_83XX_CLKIN

MPC83xx uses CONFIG_83XX_CLKIN instead of CONFIG_SYS_CLK_FREQ to set the
system clock. To migrate the architecture, we can replace
CONFIG_83XX_CLKIN with CONFIG_SYS_CLK_FREQ.

To do this
* replace all occurrences of CONFIG_83XX_CLKIN with CONFIG_SYS_CLK_FREQ
* set CONFIG_SYS_CLK_FREQ to the old value of CONFIG_83XX_CLKIN in all
  MPC83xx config files

Signed-off-by: Mario Six <mario.six@gdsys.cc>
diff --git a/Kconfig b/Kconfig
index da208d6..5f5c5cc 100644
--- a/Kconfig
+++ b/Kconfig
@@ -512,7 +512,7 @@
 
 
 config SYS_CLK_FREQ
-	depends on ARC || ARCH_SUNXI
+	depends on ARC || ARCH_SUNXI || MPC83xx
 	int "CPU clock frequency"
 	help
 	  TODO: Move CONFIG_SYS_CLK_FREQ for all the architecture