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/nokia_rx51_defconfig b/configs/nokia_rx51_defconfig
index 60bb317..82d3371 100644
--- a/configs/nokia_rx51_defconfig
+++ b/configs/nokia_rx51_defconfig
@@ -4,6 +4,7 @@
 CONFIG_ARCH_OMAP2PLUS=y
 CONFIG_SYS_TEXT_BASE=0x80008000
 CONFIG_NR_DRAM_BANKS=2
+CONFIG_SYS_MALLOC_LEN=0xc0000
 CONFIG_TARGET_NOKIA_RX51=y
 CONFIG_OPTIMIZE_INLINING=y
 CONFIG_LTO=y