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/dh_imx6_defconfig b/configs/dh_imx6_defconfig
index 8c0b9b3..a659e99 100644
--- a/configs/dh_imx6_defconfig
+++ b/configs/dh_imx6_defconfig
@@ -35,6 +35,7 @@
 CONFIG_BOOTDELAY=3
 # CONFIG_USE_BOOTCOMMAND is not set
 CONFIG_SYS_CONSOLE_OVERWRITE_ROUTINE=y
+CONFIG_SYS_SPL_MALLOC=y
 CONFIG_SPL_SPI_LOAD=y
 CONFIG_SYS_SPI_U_BOOT_OFFS=0x11400
 CONFIG_SYS_MAXARGS=32