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/mt7628_rfb_defconfig b/configs/mt7628_rfb_defconfig
index 9ee4775..640f862 100644
--- a/configs/mt7628_rfb_defconfig
+++ b/configs/mt7628_rfb_defconfig
@@ -6,6 +6,7 @@
 CONFIG_ENV_SIZE=0x1000
 CONFIG_ENV_OFFSET=0x30000
 CONFIG_ENV_SECT_SIZE=0x10000
+CONFIG_SYS_MALLOC_LEN=0x100000
 CONFIG_DEFAULT_DEVICE_TREE="mediatek,mt7628-rfb"
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_SYS_MALLOC_F_LEN=0x40000