net: ethernet: stmmac: dwmac-rk: rework optional clock handling

The clock requesting code is quite repetitive. Fix this by requesting
the clocks via devm_clk_bulk_get_optional. The optional variant has been
used, since this is effectively what the old code did. The exact clocks
required depend on the platform and configuration. As a side effect
this change adds correct -EPROBE_DEFER handling.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Suggested-by: Andrew Lunn <andrew@lunn.ch>
Fixes: 7ad269ea ("GMAC: add driver for Rockchip RK3288 SoCs integrated GMAC")
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: David S. Miller <davem@davemloft.net>