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_SDCARD_defconfig b/configs/P1020RDB-PC_36BIT_SDCARD_defconfig
index 124a9b3..ab98241 100644
--- a/configs/P1020RDB-PC_36BIT_SDCARD_defconfig
+++ b/configs/P1020RDB-PC_36BIT_SDCARD_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_OFFSET=0x0
+CONFIG_SYS_MALLOC_LEN=0x100000
 CONFIG_DEFAULT_DEVICE_TREE="p1020rdb-pc_36b"
 CONFIG_SPL_TEXT_BASE=0xf8f81000
 CONFIG_SPL_MMC_SUPPORT=y