Convert CONFIG_SYS_SPL_MALLOC_SIZE et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_SPL_MALLOC_SIZE
   CONFIG_SYS_SPL_MALLOC_START

We introduce a default value here as well, and CONFIG_SYS_SPL_MALLOC to
control if we have a malloc pool or not.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/am57xx_hs_evm_usb_defconfig b/configs/am57xx_hs_evm_usb_defconfig
index c303773..014a383 100644
--- a/configs/am57xx_hs_evm_usb_defconfig
+++ b/configs/am57xx_hs_evm_usb_defconfig
@@ -36,6 +36,8 @@
 CONFIG_ANDROID_AB=y
 CONFIG_SPL_MAX_SIZE=0x74eb0
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
+CONFIG_SYS_SPL_MALLOC=y
+CONFIG_SYS_SPL_MALLOC_SIZE=0x800000
 CONFIG_SPL_DMA=y
 CONFIG_SPL_FS_LOAD_PAYLOAD_NAME="u-boot.img"
 # CONFIG_SPL_NAND_SUPPORT is not set