docs: hwmon: add emc2305.rst to docs

Submitted to linux-hwmon mailing list at
https://patchwork.kernel.org/project/linux-hwmon/patch/20220430114905.53448-4-michaelsh@nvidia.com/



Add description of emc2305 driver.

Signed-off-by: Michael Shych <michaelsh@nvidia.com>
Reviewed-by: Vadim Pasternak <vadimp@nvidia.com>