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/pic32mzdask_defconfig b/configs/pic32mzdask_defconfig
index 0a2301d..093fa2a 100644
--- a/configs/pic32mzdask_defconfig
+++ b/configs/pic32mzdask_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SYS_MEMTEST_START=0x88000000
 CONFIG_SYS_MEMTEST_END=0x88080000
 CONFIG_ENV_SIZE=0x4000
+CONFIG_SYS_MALLOC_LEN=0x40000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="pic32mzda_sk"
 CONFIG_MACH_PIC32=y