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/pxm2_defconfig b/configs/pxm2_defconfig
index 92332de..b233995 100644
--- a/configs/pxm2_defconfig
+++ b/configs/pxm2_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SYS_MALLOC_F_LEN=0x2000
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x2000
+CONFIG_SYS_MALLOC_LEN=0x1000000
 CONFIG_SPL_DM_SPI=y
 CONFIG_DEFAULT_DEVICE_TREE="am335x-pxm50"
 CONFIG_AM33XX=y