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/qemu-riscv32_spl_defconfig b/configs/qemu-riscv32_spl_defconfig
index c836cc0..2bc7b9f 100644
--- a/configs/qemu-riscv32_spl_defconfig
+++ b/configs/qemu-riscv32_spl_defconfig
@@ -17,6 +17,7 @@
 CONFIG_DISPLAY_BOARDINFO=y
 CONFIG_SPL_MAX_SIZE=0x100000
 CONFIG_SPL_BSS_START_ADDR=0x84000000
+CONFIG_SYS_SPL_MALLOC=y
 CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 # CONFIG_CMD_MII is not set