regmap: mmio: Explicitly say little endian is the defualt in the bus config

Otherwise the DT parsing will default to big endian if nothing is
specified.

Reported-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>

drivers/base/regmap/regmap-mmio.c

···
 	.reg_write = regmap_mmio_write,
 	.reg_read = regmap_mmio_read,
 	.free_context = regmap_mmio_free_context,
+	.val_format_endian_default = REGMAP_ENDIAN_LITTLE,
 };
 
 static struct regmap_mmio_context *regmap_mmio_gen_context(struct device *dev,