Define and document 4.1 pins 48 through 54

[mciantyre]

Is your feature request related to a problem? Please describe.
The Teensy 4.1 has additional pins, numbered 48 through 54. They're used to interface external RAM and FLASH. They're not implemented in teensy4-pins.

Describe the solution you'd like
Implement and document pins 48 through 54 in the t41 module of teensy4-pins.

Describe alternatives you've considered
Directly use the imxrt-iomuxc APIs, but this requires a deeper understanding of the processor. It's more convenient to use teensy4-pins, since that's how the board identifies I/O.

Additional context

• This effort resembles Define and document SD_B0_* pins #103, which implemented pins 42 through 47.
• Here's the schematic. At a quick glance, it's not clear if these pins are documented. The official SDK might have more information.
• Using these new pins for external RAM or FLASH might not work without Teensy 4.1: extra flash, optional external RAM not addressable #86.
• I'm not sure if these pins have any / all of the relevant imxrt-iomuxc pin implementations. If they're not implemented, that could happen separately.
• Originally requested in SPI1 don't work with pins p34, p35, p37 #115.

[Alexander89]

I'm on it