gpio: mvebu: update Armada XP per-CPU comment

Commit 2233bf7a ("gpio: mvebu: switch to regmap for register access")
introduced percpu_regs to replace percpu_membase. Update the comment to
match.

Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Fixes: 2233bf7a ("gpio: mvebu: switch to regmap for register access")
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>