of/spi: Honour "status=disabled" property of device

Currently of_register_spi_devices() function registers all SPI devices,
independetly from their status property in device tree. According to
"ePAPR 1.1" spec, device should only be registered if there is no
"status" property, or it has "ok" (or "okay") value (see
of_device_is_available()). In case of "platform devices",
of_platform_device_create_pdata() checks for "status" and ensures
that disabled devices are not pupulated. But such check for SPI buses
was missing until now. Fix it.

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@sysgo.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>