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_shc_defconfig b/configs/am335x_shc_defconfig
index 66d775f..9f5924e 100644
--- a/configs/am335x_shc_defconfig
+++ b/configs/am335x_shc_defconfig
@@ -32,6 +32,8 @@
 CONFIG_DEFAULT_FDT_FILE="am335x-shc"
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
+CONFIG_SYS_SPL_MALLOC=y
+CONFIG_SYS_SPL_MALLOC_SIZE=0x800000
 CONFIG_SPL_FS_EXT4=y
 CONFIG_SPL_I2C=y
 # CONFIG_SPL_NAND_SUPPORT is not set