mfd: stm32-timers: Fix max register

STM32 timers register bank size is 0x400. Fix regmap max_register.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>