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/ti816x_evm_defconfig b/configs/ti816x_evm_defconfig
index 243492a..54c1d80 100644
--- a/configs/ti816x_evm_defconfig
+++ b/configs/ti816x_evm_defconfig
@@ -30,6 +30,8 @@
 # CONFIG_DISPLAY_BOARDINFO is not set
 # CONFIG_MISC_INIT_R is not set
 CONFIG_SPL_MAX_SIZE=0xfff1b400
+CONFIG_SYS_SPL_MALLOC=y
+CONFIG_SYS_SPL_MALLOC_SIZE=0x800000
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR=y
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR=0x300
 CONFIG_SPL_NAND_SUPPORT=y