cam_enc_4xx: Move CONFIG_SPL_PAD_TO to a config header

For most boards which define CONFIG_SPL_PAD_TO,
it is defined in config header files.
Currently, there exists only one exception, cam_enc_4xx board.

This patch moves CONFIG_SPL_PAD_TO definition
from board/ait/cam_enc_4xx/config.mk
to include/configs/cam_enc_4xx.h.

With this modification, we can delete a glue code
in the top level config.mk:

ifneq ($(CONFIG_SPL_PAD_TO),)
CPPFLAGS += -DCONFIG_SPL_PAD_TO=$(CONFIG_SPL_PAD_TO)
endif

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Heiko Schocher <hs@denx.de>