NXP LPC4088: Add missing SPI SSEL pin to Pinmap

The Pin P5_3 (p31) was missing from the NXP LPC4088 SPI PinMap for SSEL.

Adding this Pin allows usage of SPISlave with SSP2 using the SSEL pin.

The pin and its SSP2_SSEL function is both documented in https://os.mbed.com/media/uploads/embeddedartists/lpc4088_qsb_pinning.xlsx as well as in UM10562 LPC408x/407x User manual Rev. 3 — 12 March 2014 chapter 7.4.1.4 Table 90 (https://www.nxp.com/docs/en/user-guide/UM10562.pdf).

targets/TARGET_NXP/TARGET_LPC408X/TARGET_LPC4088/spi_api.c

@@ -69,6 +69,7 @@ static const PinMap PinMap_SPI_SSEL[] = {
     {P1_28, SPI_0, 5},
     {P2_23, SPI_0, 2},
     {P4_21, SPI_1, 3},
+    {P5_3,  SPI_2, 2},
     {NC   , NC   , 0}
 };