add a submodule of Renesas RX family to `hw/mcu/renesas/rx`

.gitmodules

@@ -116,3 +116,6 @@
 [submodule "hw/mcu/silabs/cmsis-dfp-efm32gg12b"]
 	path = hw/mcu/silabs/cmsis-dfp-efm32gg12b
 	url = https://github.com/cmsis-packs/cmsis-dfp-efm32gg12b
+[submodule "hw/mcu/renesas/rx"]
+	path = hw/mcu/renesas/rx
+	url = https://github.com/kkitayam/rx_device.git

hw/bsp/rx63n/boards/gr_citrus/board.mk

@@ -1,4 +1,4 @@
-DEPS_SUBMODULES += hw/mcu/renesas
+DEPS_SUBMODULES += hw/mcu/renesas/rx
 
 CFLAGS += \
   -nostartfiles \
@@ -32,7 +32,7 @@ CFLAGS += -nostdinc \
 LIBS += -loptc -loptm
 endif
 
-MCU_DIR = hw/mcu/renesas/rx63n
+MCU_DIR = hw/mcu/renesas/rx/rx63n
 
 # All source paths should be relative to the top level.
 LD_FILE = $(BOARD_PATH)/r5f5631fd.ld

hw/mcu/renesas/rx

@@ -0,0 +1 @@
+Subproject commit 4a51dfe6ecdf936d2d89f223f069e24a2d109207