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/octeontx_83xx_defconfig b/configs/octeontx_83xx_defconfig
index 7bf5c6c..3890c1e 100644
--- a/configs/octeontx_83xx_defconfig
+++ b/configs/octeontx_83xx_defconfig
@@ -8,6 +8,7 @@
 CONFIG_ENV_OFFSET=0xF00000
 CONFIG_ENV_SECT_SIZE=0x10000
 CONFIG_TARGET_OCTEONTX_83XX=y
+CONFIG_SYS_MALLOC_LEN=0x4008000
 CONFIG_DM_GPIO=y
 CONFIG_DEBUG_UART_BASE=0x87e028000000
 CONFIG_DEBUG_UART_CLOCK=24000000