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/stm32mp15_dhcom_basic_defconfig b/configs/stm32mp15_dhcom_basic_defconfig
index fa3cff0..38f1643 100644
--- a/configs/stm32mp15_dhcom_basic_defconfig
+++ b/configs/stm32mp15_dhcom_basic_defconfig
@@ -32,6 +32,10 @@
 CONFIG_SPL_LEGACY_IMAGE_FORMAT=y
 # CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
 CONFIG_SPL_STACK=0x30000000
+CONFIG_SYS_SPL_MALLOC=y
+CONFIG_HAS_CUSTOM_SPL_MALLOC_START=y
+CONFIG_CUSTOM_SYS_SPL_MALLOC_ADDR=0xc0300000
+CONFIG_SYS_SPL_MALLOC_SIZE=0x1d00000
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_PARTITION=y
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_PARTITION=3
 CONFIG_SPL_ENV_SUPPORT=y