docs: i2c: i2c-sysfs: fix hyperlinks

dts files cannot be linked conveniently, thus replace them with literal
formatting.

The links to other rst pages are broken, fix them using the proper syntax.

Fixes: 31df7195 ("Documentation: i2c: Add doc for I2C sysfs")
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>