phy: ti: am654: show up in regmap debugfs

The max_register property must be set in order to
show up the registers in debugfs.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Link: https://lore.kernel.org/r/20200513131254.10497-2-rogerq@ti.com
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>