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/bcm968360bg_ram_defconfig b/configs/bcm968360bg_ram_defconfig
index 85bae88..bdc2e1e 100644
--- a/configs/bcm968360bg_ram_defconfig
+++ b/configs/bcm968360bg_ram_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SYS_MALLOC_F_LEN=0x8000
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x2000
+CONFIG_SYS_MALLOC_LEN=0x100000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="bcm968360bg"
 CONFIG_TARGET_BCM968360BG=y