clk: imx8mn: rename vpu_pll to m7_alt_pll

[backport from linux commit a429c60baefd95ab43a2ce7f25d5b2d7a2e431df]

The IMX8MN platform does not have any video processing unit (VPU), and
indeed in the reference manual (document IMX8MNRM Rev 2, 07/2022) there
is no occurrence of its pll. From an analysis of the code and the RM
itself, I think vpu pll is used instead of m7 alternate pll, probably
for copy and paste of code taken from modules of similar architectures.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Acked-by: Sean Anderson <seanga2@gmail.com>
Link: https://lore.kernel.org/r/20221219113127.528282-2-dario.binacchi@amarulasolutions.com