MIPS: asm: uaccess: Add EVA support for str*_user operations

The str*_user functions are used to securely access NULL terminated
strings from userland. Therefore, it's necessary to use the appropriate
EVA function. However, if the string is in kernel space, then the normal
instructions are being used to access it. The __str*_kernel_asm and
__str*_user_asm symbols are the same for non-EVA mode so there is no
functional change for the non-EVA kernels.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>