soc/cores/uart: use reset_less on accumulator, reg, bitcount to reduce.

This reduces logic a bit. It does not make large difference on usual design with
only 1 UART, but is interesting on designs with hundreds of UARTs used to "document"
FPGA boards :) (similar to https://github.com/enjoy-digital/camlink_4k/blob/master/ios_stream.py)

litex/soc/cores/uart.py

@@ -35,12 +35,12 @@ def __init__(self, pads, tuning_word):
         # # #
 
         uart_clk_rxen        = Signal()
-        phase_accumulator_rx = Signal(32)
+        phase_accumulator_rx = Signal(32, reset_less=True)
 
         rx          = Signal()
         rx_r        = Signal()
-        rx_reg      = Signal(8)
-        rx_bitcount = Signal(4)
+        rx_reg      = Signal(8, reset_less=True)
+        rx_bitcount = Signal(4, reset_less=True)
         rx_busy     = Signal()
         rx_done     = self.source.valid
         rx_data     = self.source.data
@@ -87,12 +87,12 @@ def __init__(self, pads, tuning_word):
         # # #
 
         uart_clk_txen        = Signal()
-        phase_accumulator_tx = Signal(32)
+        phase_accumulator_tx = Signal(32, reset_less=True)
 
         pads.tx.reset = 1
 
-        tx_reg      = Signal(8)
-        tx_bitcount = Signal(4)
+        tx_reg      = Signal(8, reset_less=True)
+        tx_bitcount = Signal(4, reset_less=True)
         tx_busy     = Signal()
         self.sync += [
             self.sink.ready.eq(0),