digilent_arty/CRG: Add with_rst parameter to be able to easily disable rst.

On Arty, cpu_rst pin is connected to a button but also to USB-UART which also
resets the SoC when USB-UART is connected which is in some case not wanted.

with_rst provides an easy way to disable rst by setting it to False.

litex_boards/targets/digilent_arty.py

@@ -30,7 +30,7 @@ from liteeth.phy.mii import LiteEthPHYMII
 # CRG ----------------------------------------------------------------------------------------------
 
 class _CRG(Module):
-    def __init__(self, platform, sys_clk_freq):
+    def __init__(self, platform, sys_clk_freq, with_rst=True):
         self.rst = Signal()
         self.clock_domains.cd_sys       = ClockDomain()
         self.clock_domains.cd_sys4x     = ClockDomain(reset_less=True)
@@ -40,9 +40,12 @@ class _CRG(Module):
 
         # # #
 
+        clk100 = platform.request("clk100")
+        rst    = ~platform.request("cpu_reset") if with_rst else 0
+
         self.submodules.pll = pll = S7PLL(speedgrade=-1)
-        self.comb += pll.reset.eq(~platform.request("cpu_reset") | self.rst)
-        pll.register_clkin(platform.request("clk100"), 100e6)
+        self.comb += pll.reset.eq(rst | self.rst)
+        pll.register_clkin(clk100, 100e6)
         pll.create_clkout(self.cd_sys,       sys_clk_freq)
         pll.create_clkout(self.cd_sys4x,     4*sys_clk_freq)
         pll.create_clkout(self.cd_sys4x_dqs, 4*sys_clk_freq, phase=90)