ARM: dts: am57xx-beagle-x15-common: Fix wrong pinctrl selection for mmc2

Commit d20f997b ("ARM: dts: am57xx-beagle-x15: Remove pinmux
configurations for erratum i869") fat fingered a change in which
basically replaced mmc2_pinctrl_default with mmc1_pinctrl_default. And
kernel dutifully reports conflict of usage.

[...]
pinctrl-single 4a003400.pinmux: pin 4a00376c.0 already requested by
4809c000.mmc; cannot claim for 480b4000.mmc
pinctrl-single 4a003400.pinmux: pin-219 (480b4000.mmc) status -22
pinctrl-single 4a003400.pinmux: could not request pin 219 (4a00376c.0)
from group mmc1_pins_default  on device pinctrl-single
omap_hsmmc 480b4000.mmc: Error applying setting, reverse things back
omap_hsmmc 480b4000.mmc: could not initialize pin control state
[...]

But, thanks to the fact that we were in fact setting all the muxes in
U-Boot, all the MMC devices were still properly detected.

Fix the typo.

Fixes: d20f997b ("ARM: dts: am57xx-beagle-x15: Remove pinmux
configurations for erratum i869")
Reported-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
[tony@atomide.com: removed timestamps and wrapped description]
Signed-off-by: Tony Lindgren <tony@atomide.com>