Kconfig: Migrate CONFIG_NR_DRAM_BANKS

Move CONFIG_NR_DRAM_BANKS from headers to Kconfig.

Signed-off-by: Ramon Fried <ramon.fried@gmail.com>
diff --git a/Kconfig b/Kconfig
index db0f545..d96e337 100644
--- a/Kconfig
+++ b/Kconfig
@@ -104,6 +104,12 @@
 	  - CONFIG_SYS_VENDOR
 	  - CONFIG_SYS_SOC
 
+config NR_DRAM_BANKS
+	int "Number of DRAM banks"
+	default 4
+	help
+	  This defines the number of DRAM banks.
+
 config SYS_BOOT_GET_CMDLINE
 	bool "Enable kernel command line setup"
 	help