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/axs103_defconfig b/configs/axs103_defconfig
index 9ebecaa..698dbda 100644
--- a/configs/axs103_defconfig
+++ b/configs/axs103_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARC=y
 CONFIG_ISA_ARCV2=y
 CONFIG_SYS_TEXT_BASE=0x81000000
+CONFIG_SYS_MALLOC_LEN=0x0200000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="axs103"
 CONFIG_DEBUG_UART_BASE=0xe0022000