ARM: dts: am335x-sl50: Fix cannot claim requested pins for spi0

We don't need to bitbang these pins anymore, instead we muxed these
pins as SPI, after this change, done in commit 6c69f726, we introduced
the following error:

 pinctrl-single 44e10800.pinmux: pin PIN85 already requested \
 by 44e10800.pinmux; cannot claim for 48030000.spi
 pinctrl-single 44e10800.pinmux: pin-85 (48030000.spi) status -22

Fixes: 6c69f726 ("ARM: dts: am335x-sl50: Enable SPI0 interface and Flash Memory")
Cc: <stable@vger.kernel.org> # 4.11
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>