riscv: enable -fdata-sections

Enable the -fdata-sections compiler option for RISC-V. Buildman reports
the binary size decrease from this as 8365.3 bytes.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>

arch/riscv/config.mk

@@ -27,7 +27,8 @@ CONFIG_STANDALONE_LOAD_ADDR = 0x00000000
 LDFLAGS_STANDALONE += -T $(srctree)/examples/standalone/riscv.lds
 
 PLATFORM_CPPFLAGS	+= -ffixed-gp -fpic
-PLATFORM_RELFLAGS	+= -fno-common -gdwarf-2 -ffunction-sections
+PLATFORM_RELFLAGS	+= -fno-common -gdwarf-2 -ffunction-sections \
+			   -fdata-sections
 LDFLAGS_u-boot		+= --gc-sections -static -pie
 
 EFI_CRT0		:= crt0_riscv_efi.o