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/dragonboard820c_defconfig b/configs/dragonboard820c_defconfig
index 51ce75a..9d8819f 100644
--- a/configs/dragonboard820c_defconfig
+++ b/configs/dragonboard820c_defconfig
@@ -3,6 +3,7 @@
 CONFIG_SYS_TEXT_BASE=0x80080000
 CONFIG_NR_DRAM_BANKS=2
 CONFIG_ENV_SIZE=0x4000
+CONFIG_SYS_MALLOC_LEN=0x804000
 CONFIG_DEFAULT_DEVICE_TREE="dragonboard820c"
 CONFIG_TARGET_DRAGONBOARD820C=y
 CONFIG_IDENT_STRING="\nQualcomm-DragonBoard 820C"