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/am335x_guardian_defconfig b/configs/am335x_guardian_defconfig
index 33d37db28..0dc4bb2 100644
--- a/configs/am335x_guardian_defconfig
+++ b/configs/am335x_guardian_defconfig
@@ -32,6 +32,8 @@
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_ARCH_MISC_INIT=y
 # CONFIG_SPL_RAW_IMAGE_SUPPORT is not set
+CONFIG_SYS_SPL_MALLOC=y
+CONFIG_SYS_SPL_MALLOC_SIZE=0x800000
 # CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR is not set
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SPL_ETH=y