net: phy: Factor out PHY GPIO reset code

Pull the PHY GPIO reset code into separate function, since
this is and will be reused multiple times. Set up default
reset assert and deassert timing to generous 20ms and 1ms
for maximum compatibility in case those DT properties are
missing.

Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>