net: stmmac: platform: use optional clk/reset get APIs

Use the devm_reset_control_get_optional() and devm_clk_get_optional()
rather than open coding them.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20201112092606.5173aa6f@xhacker.debian
Signed-off-by: Jakub Kicinski <kuba@kernel.org>