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/socfpga_agilex_atf_defconfig b/configs/socfpga_agilex_atf_defconfig
index 5fdacb3..ebe90ce 100644
--- a/configs/socfpga_agilex_atf_defconfig
+++ b/configs/socfpga_agilex_atf_defconfig
@@ -33,6 +33,10 @@
 CONFIG_SPL_BSS_MAX_SIZE=0x100000
 # CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
 CONFIG_SPL_STACK=0xffe3f000
+CONFIG_SYS_SPL_MALLOC=y
+CONFIG_HAS_CUSTOM_SPL_MALLOC_START=y
+CONFIG_CUSTOM_SYS_SPL_MALLOC_ADDR=0x3fa00000
+CONFIG_SYS_SPL_MALLOC_SIZE=0x500000
 CONFIG_SPL_CRC32=y
 CONFIG_SPL_CACHE=y
 CONFIG_SPL_SPI_LOAD=y