arch: riscv: add config option for building SiFive's SoC resource

Create a config option for building SiFive SoC specific resources
e.g. SiFive device tree, platform drivers...

Signed-off-by: Loys Ollivier <lollivier@baylibre.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@sifive.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

arch/riscv/Kconfig

@@ -95,6 +95,8 @@ config PGTABLE_LEVELS
 	default 3 if 64BIT
 	default 2
 
+source "arch/riscv/Kconfig.socs"
+
 menu "Platform type"
 
 choice

arch/riscv/Kconfig.socs

@@ -0,0 +1,8 @@
+menu "SoC selection"
+
+config SOC_SIFIVE
+       bool "SiFive SoCs"
+       help
+         This enables support for SiFive SoC platform hardware.
+
+endmenu

arch/riscv/boot/dts/sifive/Makefile

@@ -1,2 +1,2 @@
 # SPDX-License-Identifier: GPL-2.0
-dtb-y += hifive-unleashed-a00.dtb
+dtb-$(CONFIG_SOC_SIFIVE) += hifive-unleashed-a00.dtb