I2C: support driving SCL with SDA high (slave reset)

[bugadani]

On some hardware the following function exists:

I2C_SCL_RST_SLV_EN Configures to send out SCL pulses when I2C master is IDLE. The number of
 pulses equals to I2C_SCL_RST_SLV_NUM[4:0].
 0: Invalid
 1: Send out SCL pulses
 (R/W/SC)

I2C_SCL_RST_SLV_NUM Configure the pulses of SCL generated in I2C master mode.
 Valid when I2C_SCL_RST_SLV_EN is 1.
 Measurement unit: I2C_SCLK clock cycles
 (R/W)

We may implement this as fn send_idle_clk(&mut self, pulses: usize) {} for example.

cc #2493