phy: tegra: xusb: Use a bitmask for UTMI pad power state tracking

In the Linux kernel, the following vulnerability has been resolved:

phy: tegra: xusb: Use a bitmask for UTMI pad power state tracking

The current implementation uses bias_pad_enable as a reference count to
manage the shared bias pad for all UTMI PHYs. However, during system
suspension with connected USB devices, multiple power-down requests for
the UTMI pad result in a mismatch in the reference count, which in turn
produces warnings such as:

[ 237.762967] WARNING: CPU: 10 PID: 1618 at tegra186_utmi_pad_power_down+0x160/0x170
[ 237.763103] Call trace:
[ 237.763104] tegra186_utmi_pad_power_down+0x160/0x170
[ 237.763107] tegra186_utmi_phy_power_off+0x10/0x30
[ 237.763110] phy_power_off+0x48/0x100
[ 237.763113] tegra_xusb_enter_elpg+0x204/0x500
[ 237.763119] tegra_xusb_suspend+0x48/0x140
[ 237.763122] platform_pm_suspend+0x2c/0xb0
[ 237.763125] dpm_run_callback.isra.0+0x20/0xa0
[ 237.763127] __device_suspend+0x118/0x330
[ 237.763129] dpm_suspend+0x10c/0x1f0
[ 237.763130] dpm_suspend_start+0x88/0xb0
[ 237.763132] suspend_devices_and_enter+0x120/0x500
[ 237.763135] pm_suspend+0x1ec/0x270

The root cause was traced back to the dynamic power-down changes
introduced in commit a30951d31b25 ("xhci: tegra: USB2 pad power controls"),
where the UTMI pad was being powered down without verifying its current
state. This unbalanced behavior led to discrepancies in the reference
count.

To rectify this issue, this patch replaces the single reference counter
with a bitmask, renamed to utmi_pad_enabled. Each bit in the mask
corresponds to one of the four USB2 PHYs, allowing us to track each pad's
enablement status individually.

With this change:
- The bias pad is powered on only when the mask is clear.
- Each UTMI pad is powered on or down based on its corresponding bit
in the mask, preventing redundant operations.
- The overall power state of the shared bias pad is maintained
correctly during suspend/resume cycles.

The mutex used to prevent race conditions during UTMI pad enable/disable
operations has been moved from the tegra186_utmi_bias_pad_power_on/off
functions to the parent functions tegra186_utmi_pad_power_on/down. This
change ensures that there are no race conditions when updating the bitmask.