ARM: tegra: derive CONFIG_SPL_MAX_SIZE instead of hard-coding it

ARM: tegra: derive CONFIG_SPL_MAX_SIZE instead of hard-coding it

For Tegra, the SPL and main U-Boot are concatenated together to form a
single memory image. Hence, the maximum SPL size is the different in
TEXT_BASE for SPL and main U-Boot. Instead of manually calculating
SPL_MAX_SIZE based on those two TEXT_BASE, which can lead to errors if
one TEXT_BASE is changed without updating SPL_MAX_SIZE, simply perform
the calculation automatically.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Allen Martin <amartin@nvidia.com>
Tested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>