spl: Convert CONFIG_SPL_SIZE_LIMIT to hex

spl: Convert CONFIG_SPL_SIZE_LIMIT to hex

This is currently a decimal value which is not as convenient or
meaningful. Also U-Boot tends to use hex everywhere.

Convert this option to hex and add a comment for the size_check macro.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Acked-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: correct the typo in the commit title]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>