ARM: dts: imx6ul-ccimx6ulsom: Fix the "coin" regulator name

As documented in pfuze100.yaml, the correct name for the regulator is
"coin", so change it to fix the following DT check warning:

imx6ul-ccimx6ulsbcexpress.dtb: pmic@8: regulators: 'vcoin' does not match any of the regexes:

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>