ARM: dts: imx6ul: rename mux mode name REF_CLK_32K to OSC32K_32K_OUT

This is a rebased version of patch [0].

The 32 kHz reference clock on the i.MX6UL(L) can be output by
setting the external signal XTALOSC_REF_CLK_32K in one of the
following ways [1]:

|----------------------------------------------------------|
| Signal              | Pad             | Mode | Direction |
|----------------------------------------------------------|
| XTALOSC_REF_CLK_32K | ENET1_RX_EN     | ALT2 | O         |
|                     | GPIO1_IO03      | ALT3 |           |
|                     | JTAG_TCK        | ALT6 |           |
|----------------------------------------------------------|

Before patch [2] the mux mode for the external reference clock was
missing. The patch named the mux mode as used in the NXP Linux
4.9.11_1.0.0 release, but the Reference Manual uses the name
OSC32K_32K_OUT, e.g. in [3].

As Philipp and Shawn suggest the name from the RM should be used
instead.

[0] https://patchwork.kernel.org/patch/10172187/
[1] IMX6ULRM, Rev. 1, 04/2016, Table 58-1, p. 3649
[2] https://patchwork.kernel.org/patch/10156121/


[3] IMX6ULRM, Rev. 1, 04/2016, 30.5.47 SW_MUX_CTL_PAD_ENET1_RX_EN SW MUX
    Control Register (IOMUXC_SW_MUX_CTL_PAD_ENET1_RX_EN), p. 1357

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>