update pico-serprog to Riku's new revision
    
    Riku introduced three new patches:
    
    * Add support for multiple chip selects. This allows you to
      control multiple chips from the same clip, on systems with
      dual flash setups, at least theoretically.
    * Enable pull-up on unused chip selects - pull them high so
      that chips you connect that to are deactivated while flashing
      the target chip. This could be used on thinkpad W541 for
      instance, where miso/mosi have 0ohm between them via the two
      flash ICs. You could pull the other chip select high.
    * Documentation for the above, in the pico-serprog readme.
    
    This goes in tandem with a patch from Riku, present in the
    recently integrated flashprog project, namely:
    
    commit ddb6d926783d4f9cbee04c7392718ed8f89daa0e
    Author: Riku Viitanen <riku.viitanen@protonmail.com>
    Date:   Mon Jan 15 19:15:49 2024 +0200
    
        serprog: Add support for multiple SPI chip selects
    
    This functionality will therefore be present in the next
    release of Libreboot.
    
    Signed-off-by: Leah Rowe <leah@libreboot.org>