boards: mikroe: clicker_ra4m1: added mikrobus labels

Added mikrobus_i2c, mikrobus_spi and mikrobus_serial node labels
to device tree board definition, allowing compatible shield boards
to be used. Also added User Button 2 definition and refactored
pinctrl defintions into separate file.

Signed-off-by: Ian Morris <ian.d.morris@outlook.com>
2 files changed