arm: spl: Add definition for PHY reset GPIO for XEA HW rev. 2

The imx287 based XEA board's revision 2 uses GPIO_3_21 for PHY reset

It is safe to keep the GPIO_2_13 as well, as in the SPL SPI1 is not
used for transmission. This simplifies the code, as the proper
configuration is performed either in falcon boot's read DTB or in
u-boot proper (with correct FIT configuration chosen).

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