ARM: dts: imx6sll: fixup of operating points

[ Upstream commit 18759030 ]

Make operating point definitions comply with binding
specifications.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Stable-dep-of: ee70b908 ("ARM: dts: nxp/imx6sll: fix wrong property name in usbphy node")
Signed-off-by: Sasha Levin <sashal@kernel.org>