fs: exfat: Implement trivial 'rename' support

fs: exfat: Implement trivial 'rename' support

Implement exfat_fs_rename() to rename or move files. This is used
by the 'mv' generic FS interface command. The rename implementation
for other filesystems was added recently and was not part of exfat
porting layer due to merge issue, which made 'mv' command crash,
fix this by adding the missing implementation.

Fixes: b86a651b646c ("fs: exfat: Add U-Boot porting layer")
Signed-off-by: Marek Vasut <marex@denx.de>