thermal: rcar: add read/write functions for common/priv data

R-Car thermal driver will use struct common in next
feature (interrupt support).
But the register address is different between struct priv and common.
This patch adds read/write functions for struct common,
and use macro technique to avoid wrong register access.

This is preparation patch for next feature (interrupt support),
therefore, there is no user to use this common read/write
function at this point.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>