spi/davinci: use request_threaded_irq() to fix deadlock

With RT pre-empt patch applied to Linux kernel, the irq handler will be
force converted to an irq thread. spi driver can get back to back messages
from the slave device. In such cases, IRQ thread doesn't get a chance to
run to read the slave data. Hence the irq handler must be run in hard irq
context to read/write data from slave device. Otherwise, the kernel goes
into a deadlock. This patch fixes this issue when PREEMPT_RT_FULL is
enabled in the kernel. A dummy thread function is provided to satisfy the
request_threaded_irq() API. Passing a NULL for function also causes the
irq handler to be executed in the thread context.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>