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/brppt1_nand_defconfig b/configs/brppt1_nand_defconfig
index 1db660f..59aa2cc 100644
--- a/configs/brppt1_nand_defconfig
+++ b/configs/brppt1_nand_defconfig
@@ -7,6 +7,7 @@
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x10000
 CONFIG_ENV_OFFSET=0x60000
+CONFIG_SYS_MALLOC_LEN=0x500000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="am335x-brppt1-nand"
 CONFIG_AM33XX=y