mb/siemens/mc_ehl1: Enable pi608gp I2C driver

Add devicetree and Kconfig entries to enable additional configuration
of the Pericom PI7C9X2G608GP PCIe switch on this board variant.

The amplitude is being adjusted to 425 mV and de-emphasis level to
6.0 mV.

BUG=none
TEST=Read out the PCIe config space values of the switch and check if
they match with the ones configured over SMBus.

Change-Id: I11459f0794278ad614aa6e16c56df1ad578fe2f8
Signed-off-by: Jan Samek <jan.samek@siemens.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74434
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Werner Zeh <werner.zeh@siemens.com>

src/mainboard/siemens/mc_ehl/variants/mc_ehl1/Kconfig

@@ -3,6 +3,7 @@ if BOARD_SIEMENS_MC_EHL1
 config BOARD_SPECIFIC_OPTIONS
 	def_bool y
 	select DRIVERS_I2C_RX6110SA
+	select DRIVERS_I2C_PI608GP
 	select DRIVER_INTEL_I210
 	select INTEL_LPSS_UART_FOR_CONSOLE
 	select NC_FPGA_POST_CODE

src/mainboard/siemens/mc_ehl/variants/mc_ehl1/devicetree.cb

@@ -188,6 +188,15 @@ chip soc/intel/elkhartlake
 				register "user_weekday" = "4"
 				device i2c 0x32 on end  # RTC RX6110 SA
 			end
+			# PI7C9X2G608GP PCIe switch configuration
+			chip drivers/i2c/pi608gp
+				register "gen2_3p5_enable" = "true"
+				register "gen2_3p5_amp" = "AMP_LVL_MV(425)"
+				register "gen2_3p5_deemph" = "DEEMPH_LVL_MV(6, 0)"
+				device i2c 0x6f on
+					ops pi608gp_ops
+				end
+			end
 		end
 		device pci 1f.5 on	end # PCH SPI (flash & TPM)
 	end