PCI: mediatek-gen3: Implement sys clock ready time setting

PCI: mediatek-gen3: Implement sys clock ready time setting

In preparation to add support for the PCI-Express Gen3 controller
found in newer MediaTek SoCs, such as the Dimensity 9400 MT6991
and the MT8196 Chromebook SoC, add the definition for the PCIE
Resource Control register and a new sys_clk_rdy_time_us variable
in platform data.

If sys_clk_rdy_time_us is found (> 0), set the new value in the
aforementioned register only after configuring the controller to
RC mode, as this may otherwise be reset.

Overriding the register defaults for SYS_CLK_RDY_TIME allows to
work around sys_clk_rdy signal glitching in MT6991 and MT8196.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
[mani: used FIELD_MODIFY() to simplify mask and update]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/20250703120847.121826-2-angelogioacchino.delregno@collabora.com