fix: error ATMEL_FIO_BASE undeclared, if use I2C_Soft on AT91

* Since AT91 name schema was changed to ATMEL_BASE_xxx, I2C_SOFT
  on AT91 devices fails with 'error: ATMEL_FIO_BASE undeclared'
* change ATMEL_PIO_BASE to ATMEL_BASE_PIOA will fix this

Signed-off-by: Jens Scharsig <esw@bus-elektronik.de>