platform/x86: thinkpad_acpi: Fix warning about deprecated hwmon_device_register

Use hwmon_device_register_with_groups instead of deprecated
hwmon_device_register and fix a dmesg warning.

This patch however changes the userspace API.
hwmon_device_register_with_groups takes `hwmon' name as an argument and
creates a name file in the `hwmon' device, not in the `platform_device'.
This allows us to remove custom `name' device attribute, but in order to
make lm-sensors happy we also have to move fans and thermal attributes
to the `hwmon' device.

Even though this patch changes userspace API, it's still compatible with
the lm-sensors. Starting with lm-sensors 3.0 (circa 2007), it looks at
both hwmon and the backing device for the name and other attributes.

before:
$ cat /sys/devices/platform/thinkpad_hwmon/{name,fan1_input}
thinkpad
2007
$ cat /sys/devices/platform/thinkpad_hwmon/hwmon/hwmon1/{name,fan1_input}
cat: /sys/devices/platform/thinkpad_hwmon/hwmon/hwmon1/name: No such file or directory
cat: /sys/devices/platform/thinkpad_hwmon/hwmon/hwmon1/fan1_input: No such file or directory
$ cat /sys/class/hwmon/hwmon1/{name,fan1_input}
cat: /sys/class/hwmon/hwmon1/name: No such file or directory
cat: /sys/class/hwmon/hwmon1/fan1_input: No such file or directory
$ sensors
thinkpad-isa-0000
Adapter: ISA adapter
fan1:        3533 RPM

after:
$ cat /sys/devices/platform/thinkpad_hwmon/{name,fan1_input}
cat: /sys/devices/platform/thinkpad_hwmon/name: No such file or directory
cat: /sys/devices/platform/thinkpad_hwmon/fan1_input: No such file or directory
$ cat /sys/devices/platform/thinkpad_hwmon/hwmon/hwmon1/{name,fan1_input}
thinkpad
3478
$ cat /sys/class/hwmon/hwmon1/{name,fan1_input}
thinkpad
3478
$ sensors
thinkpad-isa-0000
Adapter: ISA adapter
fan1:        3489 RPM

$ sensors -v
sensors version 3.4.0 with libsensors version 3.4.0

Signed-off-by: Stanislav Fomichev <kernel@fomichev.me>
[dvhart: cleaned up commit log, bumped version to 4.14 in the doc change]
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>