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/openpiton_riscv64_spl_defconfig b/configs/openpiton_riscv64_spl_defconfig
index 54c59a3..bf0e09b 100644
--- a/configs/openpiton_riscv64_spl_defconfig
+++ b/configs/openpiton_riscv64_spl_defconfig
@@ -28,6 +28,7 @@
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
 # CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
 CONFIG_SPL_STACK=0x83fffe80
+CONFIG_SYS_SPL_MALLOC=y
 # CONFIG_SPL_BANNER_PRINT is not set
 CONFIG_SPL_CPU=y
 CONFIG_SPL_FS_EXT4=y