add bitbang spi function - enhancement

I was measured 4.81mbps for the following code:
void shiftOutFast(byte data)
{
byte i = 8;
do{
GPIO_REG_WRITE(GPIO_OUT_W1TC_ADDRESS, 1 << CLOCK);
if(data & 0x80)
GPIO_REG_WRITE(GPIO_OUT_W1TS_ADDRESS, 1 << DATA);
else
GPIO_REG_WRITE(GPIO_OUT_W1TC_ADDRESS, 1 << DATA);
GPIO_REG_WRITE(GPIO_OUT_W1TS_ADDRESS, 1 << CLOCK);
data <<= 1;
}while(--i);
return;
}

http://nerdralph.blogspot.ca/2015/04/a-4mbps-shiftout-for-esp8266arduino.html
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