P1022DS: Set CONFIG_SPL_MAX_SIZE directly

With the u-boot-with-spl.bin rule calling $(OBJCOPY) with
CONFIG_SPL_PAD_TO, and CONFIG_SPL_PAD_TO defaulting to
CONFIG_SPL_MAX_SIZE we cannot use math here, so set it to 4096 rather
than 4 * 1024.

Signed-off-by: Tom Rini <trini@ti.com>