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/stm32h743-eval_defconfig b/configs/stm32h743-eval_defconfig
index 6157da8..042d042 100644
--- a/configs/stm32h743-eval_defconfig
+++ b/configs/stm32h743-eval_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SYS_MALLOC_F_LEN=0xF00
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x2000
+CONFIG_SYS_MALLOC_LEN=0x100000
 CONFIG_DEFAULT_DEVICE_TREE="stm32h743i-eval"
 CONFIG_STM32H7=y
 CONFIG_TARGET_STM32H743_EVAL=y