Improve GPIO default direction reset behavior

[Anubhav-30]

This PR improves the default reset behavior of the GPIO peripheral in the e203_hbirdv2 design.

Changes included:

• Updated the default value of r_gpio_dir in apb_gpio.v
• Lower 16 GPIO pins are configured as output by default (32'h0000FFFF)
• Upper 16 GPIO pins remain as input

Motivation:

• Provides a more practical default configuration for GPIO usage
• Aligns with common SoC designs where some pins are output-enabled after reset
• Improves usability without requiring immediate software configuration

Impact:

• No changes to existing APB interface or functionality
• Fully backward compatible
• Only affects initial reset state of GPIO direction

Notes:

• No modifications were made to other registers or logic
• Change is limited to reset initialization only

Tested via simulation to ensure no unintended behavior changes.

[Anubhav-30]

• Verified that only the reset default value is changed.
• No impact on existing GPIO functionality or APB interface.