drivers: counter: add driver for Microchip MCP7940N RTCC

Microchip MCP7940N is a Real-Time Clock/Calendar. It operates on a I2C
bus. It can be used to set a calendar time and has two alarm channels.
When an alarm is asserted the state of the MPF pin of the MCP7940N will
change (depending on gpio active high/active low setting) to trigger an
interrupt.

Signed-off-by: Kieran Mackey <kieran.mackey@lairdconnect.com>