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/r8a774e1_beacon_defconfig b/configs/r8a774e1_beacon_defconfig
index fc22ead..098e41f 100644
--- a/configs/r8a774e1_beacon_defconfig
+++ b/configs/r8a774e1_beacon_defconfig
@@ -3,6 +3,7 @@
 CONFIG_SYS_TEXT_BASE=0x50000000
 CONFIG_SYS_MALLOC_F_LEN=0x2000
 CONFIG_ENV_OFFSET=0x0
+CONFIG_SYS_MALLOC_LEN=0x4000000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="r8a774e1-beacon-rzg2h-kit"
 CONFIG_RCAR_GEN3=y