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/P1010RDB-PB_SDCARD_defconfig b/configs/P1010RDB-PB_SDCARD_defconfig
index 6266334..8b0ffd8 100644
--- a/configs/P1010RDB-PB_SDCARD_defconfig
+++ b/configs/P1010RDB-PB_SDCARD_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_OFFSET=0x0
+CONFIG_SYS_MALLOC_LEN=0x100000
 CONFIG_DEFAULT_DEVICE_TREE="p1010rdb-pb"
 CONFIG_SPL_TEXT_BASE=0xD0001000
 CONFIG_SPL_MMC_SUPPORT=y