ddr: fsl: Merge macro CONFIG_NUM_DDR_CONTROLLERS and CONFIG_SYS_NUM_DDR_CTRLS

These two macros are used for the same thing, the total number of DDR
controllers for a given SoC. Use SYS_NUM_DDR_CTRLS in Kconfig and
merge existing usage.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/drivers/ddr/fsl/Kconfig b/drivers/ddr/fsl/Kconfig
index b035502..a3d2bd5 100644
--- a/drivers/ddr/fsl/Kconfig
+++ b/drivers/ddr/fsl/Kconfig
@@ -23,7 +23,7 @@
 menu "Freescale DDR controllers"
 	depends on SYS_FSL_DDR
 
-config NUM_DDR_CONTROLLERS
+config SYS_NUM_DDR_CTLRS
 	int "Maximum DDR controllers"
 	default 3 if	ARCH_LS2080A	|| \
 			ARCH_T4240