PM / devfreq: rockchip-dfi: Handle LPDDR2 correctly

PM / devfreq: rockchip-dfi: Handle LPDDR2 correctly

According to the downstream driver the DDRMON_CTRL_LPDDR23 bit must be
set for both LPDDR2 and LPDDR3. Add the missing LPDDR2 case and while
at it turn the if/else if/else into switch/case which makes it easier
to read.

Link: https://lore.kernel.org/all/20231018061714.3553817-12-s.hauer@pengutronix.de/
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>