pwm: axi-pwmgen: add .max_register to regmap

pwm: axi-pwmgen: add .max_register to regmap

This was missed in the basic driver and is useful for debug, so add it.

Example regmap output before the patch:

|root@zed-tg:~# cat /sys/kernel/debug/regmap/44a60000.pwm/registers
|0: 00020100

And with it:

|root@zed-tg:~# cat /sys/kernel/debug/regmap/44a60000.pwm/registers
|00: 00020100
|04: 00000000
|08: 00000000
|0c: 601a3471
|10: 00000000
|14: 00000002
|18: 00000001
|1c: 00000000
|...

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Link: https://lore.kernel.org/r/20240711125743.3956935-1-tgamblin@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>