Nokia RX-51: Move board required options from defconfig to Kconfig

Some of config options are board specific and should be set in into their
default values automatically. So move them from defconfig file to Kconfig
definitions to ensure that possible user custom defconfig files would have
these required options also enabled.

Signed-off-by: Pali Rohár <pali@kernel.org>
diff --git a/board/nokia/rx51/Kconfig b/board/nokia/rx51/Kconfig
index ec6a571..7cf0507 100644
--- a/board/nokia/rx51/Kconfig
+++ b/board/nokia/rx51/Kconfig
@@ -1,5 +1,8 @@
 if TARGET_NOKIA_RX51
 
+config NR_DRAM_BANKS
+	default 2
+
 config SYS_BOARD
 	default "rx51"
 
@@ -9,4 +12,7 @@
 config SYS_CONFIG_NAME
 	default "nokia_rx51"
 
+config SYS_PROMPT
+	default "Nokia RX-51 # "
+
 endif