spim_nrf52: Prevent glitch on CS line in spim_nrf52_init()

The output state of the CS GPIO must be configured with a pull-up while
setting the GPIO as output. Otherwise the GPIO will be forced low,
before being set high by the call to spim_nrf52_csn(). This results in a
glitch of 1us on the CS line, which may confuse some ICs in the worst
case.

Signed-off-by: Florian Vaussard <florian.vaussard@gmail.com>