phy: amlogic: phy-meson-gxl-usb2: rename some of the U2P_R2 registers

The U2P_R2 register provides "test mode" functionality for bits 17:0.
These are only used during SoC development and should be left untouched
on production SoC versions.
Rename these register definitions to indicate that these are for "test
mode" only.

While here, also merge the definitions for U2P_R2_DATA_IN_MASK and
U2P_R2_DATA_IN_EN_MASK (bits 0:7) because Amlogic's internal
documentation suggests that these bits belong together. The old
definition was not taken from the documentation but rather from a struct
definition in the Amlogic GPL kernel sources.

No functional changes.

Suggested-by: Yixun Lan <yixun.lan@amlogic.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>