powerpc: mpc86xx: Move CONFIG_FSL_LAW to Kconfig

Clean up existing definitions and drop from white list.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/arch/powerpc/cpu/mpc86xx/Kconfig b/arch/powerpc/cpu/mpc86xx/Kconfig
index 57e7476..42b2ed2 100644
--- a/arch/powerpc/cpu/mpc86xx/Kconfig
+++ b/arch/powerpc/cpu/mpc86xx/Kconfig
@@ -28,9 +28,16 @@
 
 config ARCH_MPC8610
 	bool
+	select FSL_LAW
 
 config ARCH_MPC8641
 	bool
+	select FSL_LAW
+
+config FSL_LAW
+	bool
+	help
+		Use Freescale common code for Local Access Window
 
 config SYS_CCSRBAR_DEFAULT
 	hex "Default CCSRBAR address"