pmic: dm: Rewrite pmic_reg_{read|write|clrsetbits} to support 3 bytes transmissions

This commit provides support for transmissions larger than 1 byte for
PMIC devices used with DM (e.g. MC34708 from NXP).

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>