mpc83xx: Migrate CONFIG_SYS_IMMR to Kconfig

Migrate CONFIG_SYS_IMMR to Kconfig for MPC83xx.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
diff --git a/configs/TQM834x_defconfig b/configs/TQM834x_defconfig
index e85bfae..f5746cf 100644
--- a/configs/TQM834x_defconfig
+++ b/configs/TQM834x_defconfig
@@ -4,6 +4,7 @@
 CONFIG_MPC83xx=y
 CONFIG_HIGH_BATS=y
 CONFIG_TARGET_TQM834X=y
+CONFIG_SYS_IMMR=0xff400000
 CONFIG_DDR_MC_CLOCK_MODE_1_1=y
 CONFIG_SYSTEM_PLL_FACTOR_4_1=y
 CONFIG_CORE_PLL_RATIO_2_1=y