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/configs/kmtepr2_defconfig b/configs/kmtepr2_defconfig
index 310cf5a..bbd13b8 100644
--- a/configs/kmtepr2_defconfig
+++ b/configs/kmtepr2_defconfig
@@ -1,5 +1,6 @@
 CONFIG_PPC=y
 CONFIG_SYS_TEXT_BASE=0xF0000000
+CONFIG_SYS_CLK_FREQ=66000000
 CONFIG_MPC83xx=y
 CONFIG_TARGET_KMTEPR2=y
 CONFIG_OF_BOARD_SETUP=y