riscv: add dummy eth

* it compiles, but cannot allocate dma buffer at runtime

arch/riscv/Kconfig

@@ -222,6 +222,8 @@ endmenu
 
 menu "Bus support"
 
+source "arch/riscv/drivers/Kconfig"
+
 config PCI
 	bool "PCI support"
 	select PCI_MSI

arch/riscv/Makefile

@@ -80,4 +80,6 @@ core-y += arch/riscv/kernel/ arch/riscv/mm/
 
 libs-y += arch/riscv/lib/
 
+drivers-y += arch/riscv/drivers/
+
 all: vmlinux

arch/riscv/drivers/Kconfig

+config ETH_XILINX_RV
+	bool "Xilinx AXI dummy eth support for RV"
+	depends on RISCV
+	help
+  Say 'Y' here if you want kernel to support the Xilinx AXI dummy eth driver.

arch/riscv/drivers/Makefile

+obj-$(CONFIG_ETH_XILINX_RV) += xilinx_dummy_eth_main.o