MAINTAINERS: Add entry for ROHM Power Management ICs

Add entry for maintaining power management IC drivers for ROHM
BD71837, BD71847, BD71850, BD71828, BD71878, BD70528 and BD99954.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Acked-by: Sebastian Reichel <sre@kernel.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>