spi/ep93xx: clean probe/remove routines

Use devm_* functions for managing devres resources.

Also use local variable irq and remove irq variable from
struct ep93xx_spi, as it is needed only in probe routine.

Acked-by: Mika Westerberg <mika.westerberg@iki.fi>
Acked-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Hannu Heikkinen <hannuxx@iki.fi>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>