clk: imx8mn: rename vpu_pll to m7_alt_pll

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.

As an example for all, if we consider the second row of the "Clock Root"
table of chapter 5 (Clocks and Power Management) of the RM:

     Clock Root     offset     Source Select (CCM_TARGET_ROOTn[MUX])
        ...          ...                    ...
  ARM_M7_CLK_ROOT   0x8080            000 - 24M_REF_CLK
                                      001 - SYSTEM_PLL2_DIV5
				      010 - SYSTEM_PLL2_DIV4
				      011 - M7_ALT_PLL_CLK
				      100 - SYSTEM_PLL1_CLK
				      101 - AUDIO_PLL1_CLK
				      110 - VIDEO_PLL_CLK
				      111 - SYSTEM_PLL3_CLK
        ...          ...                    ...

but in the source code, the imx8mn_m7_sels clocks list contains vpu_pll
for the source select bits 011b.

So, let's rename "vpu_pll" to "m7_alt_pll" to be consistent with the RM.

The IMX8MN_VPU_* constants have not been removed to ensure backward
compatibility of the patch.

No functional changes intended.

Fixes: 96d6392b ("clk: imx: Add support for i.MX8MN clock driver")
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Acked-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Abel Vesa <abel.vesa@linaro.org>
Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
Link: https://lore.kernel.org/r/20221117113637.1978703-2-dario.binacchi@amarulasolutions.com