drivers: richtek: fix write-out-of-bounds in rt_regmap_cache_init()

KASAN warns about a write-out-of-bounds in rt_regmap_cache_init():

	if (!rd->props.group) {
		rd->props.group = devm_kzalloc(&rd->dev,
				sizeof(rd->props.group), GFP_KERNEL);   <- allocated here
		rd->props.group[0].start = 0x00;
		rd->props.group[0].end = 0xffff;
		rd->props.group[0].mode = RT_1BYTE_MODE;                <- written here
	}

The devm_kzalloc() call a few lines above is accidentally requesting
enough space to store a pointer type, which isn't enough space to hold
the struct itself.

Change-Id: I0036262b3129bd86d2e8612fb9b67a848bbb4ead
Signed-off-by: Greg Hackmann <ghackmann@google.com>