media: imx: imx7_mipi_csis: Define macros for DPHY_BCTRL_L fields

Define macros for the fields of the MIPI_CSIS_DPHY_BCTRL_L register,
based on the i.MX8MM datasheet. The numerical value written to the
register, taken from the i.MX7D code, is unchanged, and corresponds to a
20.0MHz Tx escape clock frequency according to the documentation of the
register from the i.MX8MM. Whether the other fields are applicable to
the i.MX7D is unknown.

While at it, rename the DPHY[BS]CTRL register names to DPHY_[BS]CTRL to
match the datasheet.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>