Convert CONFIG_SPL_PAD_TO et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_PAD_TO
   CONFIG_SPL_MAX_SIZE
   CONFIG_TPL_PAD_TO
   CONFIG_TPL_MAX_SIZE

Note that we need to make TPL_MAX_SIZE be hex, and so move and convert the
existing places.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/imx8mn_ddr4_evk_defconfig b/configs/imx8mn_ddr4_evk_defconfig
index 6f96f3a..c62b193 100644
--- a/configs/imx8mn_ddr4_evk_defconfig
+++ b/configs/imx8mn_ddr4_evk_defconfig
@@ -25,6 +25,7 @@
 CONFIG_OF_SYSTEM_SETUP=y
 CONFIG_DEFAULT_FDT_FILE="imx8mn-ddr4-evk.dtb"
 CONFIG_BOARD_LATE_INIT=y
+CONFIG_SPL_MAX_SIZE=0x25000
 CONFIG_SPL_BOARD_INIT=y
 CONFIG_SPL_BOOTROM_SUPPORT=y
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR=y