net: stmmac: dwmac-stm32: refactor clock config

Currently, clock configuration is spread throughout the driver and
partially duplicated for the STM32MP1 and STM32 MCU variants. This makes
it difficult to keep track of which clocks need to be enabled or disabled
in various scenarios.

This patch adds symmetric stm32_dwmac_clk_enable/disable() functions
that handle all clock configuration, including quirks required while
suspending or resuming. syscfg_clk and clk_eth_ck are not present on
STM32 MCUs, but it is fine to try to configure them anyway since NULL
clocks are ignored.

Signed-off-by: Ben Wolsieffer <ben.wolsieffer@hefring.com>
Signed-off-by: David S. Miller <davem@davemloft.net>