ar71xx: export some clock functions used by some modules

ar71xx: export some clock functions used by some modules

This fixes this linking problem:

ERROR: "clk_set_rate" [drivers/usb/phy/phy-generic.ko] undefined!
ERROR: "clk_round_rate" [drivers/media/v4l2-core/videodev.ko] undefined!
ERROR: "clk_set_rate" [drivers/media/v4l2-core/videodev.ko] undefined!

In the upstream kernel it is fixed here:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=411520af8ec9456886359b42628e583ac58e7e44

This closes: #20162, #20172, #20196, #20197, #20202 and #20308

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46572 3c298f89-4303-0410-b956-a3cf2f4a3e73