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/myir_mys_6ulx_defconfig b/configs/myir_mys_6ulx_defconfig
index f5ccf66..f748b4d 100644
--- a/configs/myir_mys_6ulx_defconfig
+++ b/configs/myir_mys_6ulx_defconfig
@@ -19,6 +19,7 @@
 CONFIG_FIT=y
 CONFIG_BOOTDELAY=3
 CONFIG_BOARD_EARLY_INIT_F=y
+CONFIG_SYS_SPL_MALLOC=y
 CONFIG_SPL_DMA=y
 CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_SPL_USB_HOST=y