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/at91sam9g20ek_nandflash_defconfig b/configs/at91sam9g20ek_nandflash_defconfig
index 9890b25..3c4e847 100644
--- a/configs/at91sam9g20ek_nandflash_defconfig
+++ b/configs/at91sam9g20ek_nandflash_defconfig
@@ -6,6 +6,7 @@
 CONFIG_TARGET_AT91SAM9260EK=y
 CONFIG_SYS_MALLOC_F_LEN=0x2000
 CONFIG_NR_DRAM_BANKS=1
+CONFIG_SYS_MALLOC_LEN=0x80000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="at91sam9g20ek"
 CONFIG_DEBUG_UART_BOARD_INIT=y