clk: imx8mn: fix imx8mn_sai2_sels clocks list

According to the "Clock Root" table of the reference manual (document
IMX8MNRM Rev 2, 07/2022):

     Clock Root     offset     Source Select (CCM_TARGET_ROOTn[MUX])
        ...          ...                    ...
   SAI2_CLK_ROOT    0xA600            000 - 24M_REF_CLK
                                      001 - AUDIO_PLL1_CLK
                                      010 - AUDIO_PLL2_CLK
                                      011 - VIDEO_PLL_CLK
                                      100 - SYSTEM_PLL1_DIV6
                                      110 - EXT_CLK_2
                                      111 - EXT_CLK_3
        ...          ...                    ...

while the imx8mn_sai2_sels list contained clk_ext3 and clk_ext4 for
source select bits 110b and 111b.

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/20221117113637.1978703-5-dario.binacchi@amarulasolutions.com