thermal: broadcom: add Northstar thermal driver

Northstar is a SoC family commonly used in home routers. This commit
adds a driver for checking CPU temperature. As Northstar Plus seems to
also have this IP block this new symbol gets ARCH_BCM_IPROC dependency.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Jon Mason <jon.mason@broadcom.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>