clkdev: Move clk creation outside of 'clocks_mutex'

We don't need to hold the 'clocks_mutex' here when we're creating a clk
pointer from a clk_lookup structure. Instead, we just need to make sure
that the lookup doesn't go away while we dereference the lookup pointer
to extract the clk_hw pointer out of it. Let's move things around
slightly so that we have a new function to get the clk_hw out of the
lookup with the right locking and then chain the two together for what
used to be __clk_get_sys().

Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Jerome Brunet <jbrunet@baylibre.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Jeffrey Hugo <jhugo@codeaurora.org>
Cc: Chen-Yu Tsai <wens@csie.org>
Tested-by: Jeffrey Hugo <jhugo@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>