[Linux]: use shared pin FD cache

[2bndy5]

resolves #1026

[2bndy5]

I realize now that this lib has no need for digitalRead(). I discovered this in developing another lib that does use digitalRead() (on an IRQ pin) and uses a similar version of this SPIDEV code... I'm just submitting it here in case any users get clever enough to

pinMode(IRQ_PIN, INPUT);
attachInterrupt(IRQ_PIN, FALLING, &isrFunc);
bool isInactive = GPIO::read(IRQ_PIN);

on Linux with the SPIDEV driver.

[TMRh20]

Tested & functional on RPi4 & 5