[LPC11XX_11CXX] Fix SPI slave issue

- Fix a bug reported here:
https://developer.mbed.org/questions/4872/SPISlave-Class-dosnt-work-on-LPC1114/
- Add IOCON settings to enable proper pin functions
- Remove non-busy check in slave_receive() function, since the SSP/SPI
is always in busy state when received a valid data according to the
device user manual
- Add bit mask in spi_slave_read() function
- Add TARGET_LPC1114 pin config in SPI_SLAVE test
- Tested with LPC11U24 SPI master and LPC1114 SPI slave