dt-bindings: mfd: syscon: Document the GPR syscon for the NXP S32 SoCs

dt-bindings: mfd: syscon: Document the GPR syscon for the NXP S32 SoCs

The NXP S32 SoCs have a GPR region which is used by a variety of
drivers.  Some examples of the registers in this region are:

  * DDR_PMU_IRQ
  * GMAC0_PHY_INTF_SEL
  * GMAC1_PHY_INTF_SEL
  * PFE_EMACS_INTF_SEL
  * PFE_COH_EN
  * PFE_PWR_CTRL
  * PFE_EMACS_GENCTRL1
  * PFE_GENCTRL3

Use the syscon interface to access these registers.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://patch.msgid.link/792d3f59b9f519529b94e673faf70d77c4b61fb3.1765806521.git.dan.carpenter@linaro.org
Signed-off-by: Lee Jones <lee@kernel.org>