FIX: Change default UART FIFO size and combine UART Config fragments

[T-K-233]

Many tapeout chips are using the default WithUART config in the AbstractConfig. In WithUART, the TX and RX FIFO depth is set to 256, which is an unrealistically large value. Moreover, it makes UART debugging very difficult, since the FIFO is not flushed when chip is under reset.

This PR changes the default FIFO depth to 8, which is the value used on FE310.

Additionally, since now we have control over FIFO depth in the WithUART config fragment, the WithUARTFIFOEntries is redundant. This PR changes the code that references WithUARTFIFOEntries to WithUART.

The WithUARTFIFOEntries is marked deprecated, and is recommended to be removed in future releases.

Related PRs / Issues:

Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• RTL change
• Software change (RISC-V software)
• Build system change
• Other

Contributor Checklist:

• Did you set main as the base branch?
• Is this PR's title suitable for inclusion in the changelog and have you added a changelog:<topic> label?
• Did you state the type-of-change/impact?
• Did you delete any extraneous prints/debugging code?
• Did you mark the PR with a changelog: label?
• (If applicable) Did you update the conda .conda-lock.yml file if you updated the conda requirements file?
• (If applicable) Did you add documentation for the feature?
• (If applicable) Did you add a test demonstrating the PR?

• (If applicable) Did you mark the PR as Please Backport?

[jerryz123]

@abejgonzalez Does this have any implications for Firesim?

[abejgonzalez]

SFTM.