mips: add __image_copy_len for SPL linker script

This patch adds __image_copy_len needed by TPL of MT7621 SoC.
The __image_copy_len represents the binary blob size of both SPL/TPL
binaries. To achieve this, __text_start/end are added for calculation.

Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>