dts: bindings: lis2dh: create a common st,lis2dh-common.yaml

Create a common binding file that will be included by all bindings
handled by lis2dh.c driver. For now this includes optional irq-gpios
property.

Use introduced st,lis2dh-common.yaml in st,lsm303dlhc-accel.yaml in
order to support defining irq-gpios.

Also improve description of st,lis2dh-i2c.yaml to better match what can
be found in st,lis2dh-spi.yaml.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>