spi: dw: use managed resources

Migrate mmio code and core driver to managed resources to reduce boilerplate
error handling code. Also, handle clk_enable() failure while at it, and drop
unused dw_spi iolen field.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Mark Brown <broonie@linaro.org>