mfd: Add ingenic-tcu.h header

This header contains macros for the registers that are present in the
regmap shared by all the drivers related to the TCU (Timer Counter Unit)
of the Ingenic JZ47xx SoCs.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>