regmap: Add support for 12/20 register formatting

Devices such as the AD5628 require 32 bits of data divided in 12 bits
for dummy, command and address, and 20 for data and dummy. Eg:

XXXXCCCCAAAADDDDDDDDDDDDDDDDXXXX

Where X is dont care, C is command, A is address and D is data bits.

Which would requierd the following regmap_config:

static const struct regmap_config config_dac = {
	.reg_bits = 12,
	.val_bits = 20,
	.max_register = 0xff,
};
Signed-off-by: NRicardo Ribalda <ribalda@kernel.org>
Link: https://lore.kernel.org/r/20200917114727.1120373-1-ribalda@kernel.orgSigned-off-by: NMark Brown <broonie@kernel.org>

drivers/base/regmap/regmap.c

@@ -209,6 +209,18 @@ static bool regmap_volatile_range(struct regmap *map, unsigned int reg,
 	return true;
 }
 
+static void regmap_format_12_20_write(struct regmap *map,
+				     unsigned int reg, unsigned int val)
+{
+	u8 *out = map->work_buf;
+
+	out[0] = reg >> 4;
+	out[1] = (reg << 4) | (val >> 16);
+	out[2] = val >> 8;
+	out[3] = val;
+}
+
+
 static void regmap_format_2_6_write(struct regmap *map,
 				     unsigned int reg, unsigned int val)
 {
@@ -870,6 +882,16 @@ struct regmap *__regmap_init(struct device *dev,
 		}
 		break;
 
+	case 12:
+		switch (config->val_bits) {
+		case 20:
+			map->format.format_write = regmap_format_12_20_write;
+			break;
+		default:
+			goto err_hwlock;
+		}
+		break;
+
 	case 8:
 		map->format.format_reg = regmap_format_8;
 		break;