drivers: flash: nrf_qspi_nor: Handle properly multiple XIP users

Add reference counting in nrf_qspi_nor_xip_enable() so that XIP is
kept enabled as long as there is at least one user that needs it
(boot time enabling done with CONFIG_NORDIC_QSPI_NOR_XIP also counts).

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>