arm, davinci: Use CONFIG_SPL_PAD_TO for padding the SPL in an ais image

The commits

commit b7b5f1a1
Author: Albert ARIBAUD <albert.u.boot@aribaud.net>
da850evm, da850_am18xxevm: convert to CONFIG_SPL_MAX_FOOTPRINT

and

commit e7497891
Author: Albert ARIBAUD <albert.u.boot@aribaud.net>
cam_enc_4xx: convert to CONFIG_SPL_MAX_FOOTPRINT

replaced CONFIG_SPL_MAX_SIZE by CONFIG_SPL_MAX_FOOTPRINT. However,
CONFIG_SPL_MAX_SIZE is used in the Makefile for padding the SPL
when preparing an u-boot.ais image. By removing CONFIG_SPL_MAX_SIZE
said commits broke the ais image of the da850evm and cam_enc_4xx
configurations.

This patch converts the u-boot.ais target to use CONFIG_SPL_PAD_TO
instead of CONFIG_SPL_MAX_SIZE for padding the SPL and adds
a #define CONFIG_SPL_PAD_TO where it is required.

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Reported-by: Tom Taylor <ttaylor.tampa@gmail.com>
Cc: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>