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/imx8mq_phanbell_defconfig b/configs/imx8mq_phanbell_defconfig
index c263490..427beb9 100644
--- a/configs/imx8mq_phanbell_defconfig
+++ b/configs/imx8mq_phanbell_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SYS_I2C_MXC_I2C1=y
 CONFIG_SYS_I2C_MXC_I2C2=y
 CONFIG_SYS_I2C_MXC_I2C3=y
+CONFIG_SYS_MALLOC_LEN=0x600000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="imx8mq-phanbell"
 CONFIG_SPL_TEXT_BASE=0x7E1000