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/pico-imx7d_defconfig b/configs/pico-imx7d_defconfig
index 50eb3c4..896de9f 100644
--- a/configs/pico-imx7d_defconfig
+++ b/configs/pico-imx7d_defconfig
@@ -10,6 +10,7 @@
 CONFIG_SYS_I2C_MXC_I2C2=y
 CONFIG_SYS_I2C_MXC_I2C3=y
 CONFIG_SYS_I2C_MXC_I2C4=y
+CONFIG_SYS_MALLOC_LEN=0x2000000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="imx7d-pico-pi"
 CONFIG_TARGET_PICO_IMX7D=y