Fix for GPIO0/2/4/5 not being able to be used as inputs (GIT8266O-649)

[amakukha]

After migrating my project from Arduino to ESP SDK, I noticed that GPIO2 couldn't work as input anymore. I tried to upload the basic GPIO example with GPIO2 being the input and it didn't work. After some investigating, it turned out that the GPIO driver has a problem: it treats pins GPIO0/2/4/5 as RTC pins when setting the GPIO function bits, despite there being only one RTC pin according to the ESP8266 pin list – RTC_GPIO0.

This is a fix for this problem.

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