[MIPS] unexport copy_to_user_page()

The copy_to_user_page() function is called only in the core kernel
code. Therefore, there is no need to export it. This patch removes
EXPORT_SYMBOL(copy_to_user_page).

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>