net: stmmac: thead: Enable TX clock before MAC initialization

The clk_tx_i clock must be supplied to the MAC for successful
initialization. On TH1520 SoC, the clock is provided by an internal
divider configured through GMAC_PLLCLK_DIV register when using RGMII
interface. However, currently we don't setup the divider before
initialization of the MAC, resulting in DMA reset failures if the
bootloader/firmware doesn't enable the divider,

[    7.839601] thead-dwmac ffe7060000.ethernet eth0: Register MEM_TYPE_PAGE_POOL RxQ-0
[    7.938338] thead-dwmac ffe7060000.ethernet eth0: PHY [stmmac-0:02] driver [RTL8211F Gigabit Ethernet] (irq=POLL)
[    8.160746] thead-dwmac ffe7060000.ethernet eth0: Failed to reset the dma
[    8.170118] thead-dwmac ffe7060000.ethernet eth0: stmmac_hw_setup: DMA engine initialization failed
[    8.179384] thead-dwmac ffe7060000.ethernet eth0: __stmmac_open: Hw setup failed

Let's simply write GMAC_PLLCLK_DIV_EN to GMAC_PLLCLK_DIV to enable the
divider before MAC initialization. Note that for reconfiguring the
divisor, the divider must be disabled first and re-enabled later to make
sure the new divisor take effect.

The exact clock rate doesn't affect MAC's initialization according to my
test. It's set to the speed required by RGMII when the linkspeed is
1Gbps and could be reclocked later after link is up if necessary.

Fixes: 33a1a01e ("net: stmmac: Add glue layer for T-HEAD TH1520 SoC")
Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Drew Fustini <fustini@kernel.org>
Link: https://patch.msgid.link/20250815104803.55294-1-ziyao@disroot.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>