defconfig: xea: Change default spi-nor memory bus to 2 (single binary)

After the re-sync with Linux kernel (v6.0) of the XEA DTS
(SHA1: 7d08ddd0) the alias
for SPI bus, to which SPI-NOR  memory is connected, has changed from
'spi3' to 'spi2'.

To be in sync with current u-boot's xea dts, the default bus number
(which allows running 'sf probe' without any extra parameters given)
has been adjusted.

Signed-off-by: Lukasz Majewski <lukma@denx.de>