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/warp_defconfig b/configs/warp_defconfig
index 8ae96fd..015a153 100644
--- a/configs/warp_defconfig
+++ b/configs/warp_defconfig
@@ -8,6 +8,7 @@
 CONFIG_TARGET_WARP=y
 CONFIG_SYS_I2C_MXC_I2C1=y
 CONFIG_SYS_I2C_MXC_I2C2=y
+CONFIG_SYS_MALLOC_LEN=0x2300000
 # CONFIG_CMD_BMODE is not set
 CONFIG_SUPPORT_RAW_INITRD=y
 CONFIG_BOOTDELAY=3