arm64/mm: Fixed some coding style issues

Adjust whitespace for fixmap_pXd() functions returning pointers for
consistency with the kernel coding style.

Signed-off-by: Zhiyuan Dai <daizhiyuan@phytium.com.cn>
Link: https://lore.kernel.org/r/1613958231-5474-1-git-send-email-daizhiyuan@phytium.com.cn


Signed-off-by: Will Deacon <will@kernel.org>