Convert CONFIG_SYS_MALLOC_LEN to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MALLOC_LEN

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
diff --git a/configs/P1020RDB-PC_36BIT_NAND_defconfig b/configs/P1020RDB-PC_36BIT_NAND_defconfig
index 3b62757..5e9a0a2 100644
--- a/configs/P1020RDB-PC_36BIT_NAND_defconfig
+++ b/configs/P1020RDB-PC_36BIT_NAND_defconfig
@@ -2,6 +2,7 @@
 CONFIG_SYS_TEXT_BASE=0x11001000
 CONFIG_ENV_SIZE=0x4000
 CONFIG_ENV_OFFSET=0x100000
+CONFIG_SYS_MALLOC_LEN=0x100000
 CONFIG_DEFAULT_DEVICE_TREE="p1020rdb-pc_36b"
 CONFIG_SPL_TEXT_BASE=0xFF800000
 CONFIG_SPL_SERIAL_SUPPORT=y