driver: gpio: remove nano_timer code

drivers/gpio/gpio_pcal9535a.c defines a nano_timer,
but never uses it. So delete it.
gpio_sch.c uses a k_timer, but gpio_sch.h defines a
nano_timer. So change the variable poll_timer to
type k_timer.

Jira: ZEP-1525

Change-Id: I884e4703c1ace61da5be7d9c63e58e7c3bce7f68
Signed-off-by: Qiu Peiyang <peiyangx.qiu@intel.com>