riscv: Enlarge the default SYS_MALLOC_F_LEN

Increase the heap size for the pre-relocation stage, so that CPU
driver can be loaded.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Anup Patel <anup@brainfault.org>
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 39ca2d8..c45e4d7 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -113,4 +113,7 @@
 	  standard rdtime instruction. This is the case for S-mode U-Boot, and
 	  is useful for processors that support rdtime in M-mode too.
 
+config SYS_MALLOC_F_LEN
+	default 0x1000
+
 endmenu