Fix ESP8266 GPIO0 Pullup Validation

[OttoWinter]

What does this implement/fix?

ESP8266 does have a internal pullup resistors on pin GPIO0 that can
be toggled. But GPIO16 doesn't have any. I'm not sure whether there was a reason for this,
but from all reference material I could find it's like this:

• GPIO 0-15 support internal pullups, but not pulldowns
• GPIO 16 supports internal pulldown, but not pullup

Worth mentioning however is that GPIO0 usually has additional pullups anyway, because that's one of the strapping pins during boot and must be held high during that time.

Section 4.1 ESP8266EX Datasheet: https://espressif.com/sites/default/files/documentation/0a-esp8266ex_datasheet_en.pdf
TTAPA github guide: https://tttapa.github.io/ESP8266/Chap04%20-%20Microcontroller.html

Fixes esphome/issues#2574

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

Related issue or feature (if applicable): fixes

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#

Test Environment

• ESP32
• ESP32 IDF
• ESP8266

Example entry for config.yaml:

# Example config.yaml

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[probot-esphome]

Hey there @esphome/core, mind taking a look at this pull request as it has been labeled with an integration (esp8266) you are listed as a code owner for? Thanks!
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