regulator/rpi-panel-attiny: Use the regmap cache

The I2C to the Atmel is very fussy, and locks up easily on
Pi0-3 particularly on reads.

The LCD power status is controlled solely by this driver, so
rather than reading it back from the Atmel, use the regmap cache
to avoid reading values.
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
Link: https://lore.kernel.org/r/20220124220129.158891-9-detlev.casanova@collabora.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/regulator/rpi-panel-attiny-regulator.c

@@ -76,7 +76,7 @@ static const struct regmap_config attiny_regmap_config = {
 	.val_bits = 8,
 	.disable_locking = 1,
 	.max_register = REG_WRITE_DATA_L,
-	.cache_type = REGCACHE_NONE,
+	.cache_type = REGCACHE_RBTREE,
 };
 
 static int attiny_set_port_state(struct attiny_lcd *state, int reg, u8 val)