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/gwventana_emmc_defconfig b/configs/gwventana_emmc_defconfig
index f0e7204..ef2ee77 100644
--- a/configs/gwventana_emmc_defconfig
+++ b/configs/gwventana_emmc_defconfig
@@ -41,6 +41,7 @@
 CONFIG_PCI_INIT_R=y
 CONFIG_SPL_BOARD_INIT=y
 CONFIG_SPL_STACK_R=y
+CONFIG_SYS_SPL_MALLOC=y
 CONFIG_SPL_FIT_IMAGE_TINY=y
 CONFIG_SPL_DMA=y
 CONFIG_SPL_I2C=y