arm64: change ARCH_SPRD Kconfig to tristate

The default value of Kconfig for almost all sprd drivers are the same with
ARCH_SPRD, making these drivers built as modules as default would be easier
if we can set ARCH_SPRD as 'm', so this patch change ARCH_SPRD to tristate.
Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
Link: https://lore.kernel.org/r/20200305103228.9686-1-zhang.lyra@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/arm64/Kconfig.platforms

@@ -249,7 +249,7 @@ config ARCH_TEGRA
 	  This enables support for the NVIDIA Tegra SoC family.
 
 config ARCH_SPRD
-	bool "Spreadtrum SoC platform"
+	tristate "Spreadtrum SoC platform"
 	help
 	  Support for Spreadtrum ARM based SoCs