Hi Sweta,
I was able to verify that in pinmuxing mode 3, the I2C clock and data signals are produced on pins D15 and D16. I suspect that your pinmuxing setup may be the issue since you were able to bitbang these pins as GPIO. For the pinmuxing setup I used the code below:
/* I2C_SCLK */
HWREG(SOC_CONTROL_REGS + CONTROL_CONF_UART_TXD(1)) =
(CONTROL_CONF_UART1_TXD_CONF_UART1_TXD_PUTYPESEL |
CONTROL_CONF_UART1_TXD_CONF_UART1_TXD_RXACTIVE |
CONTROL_CONF_UART1_TXD_CONF_UART1_TXD_SLEWCTRL |
CONTROL_CONF_MUXMODE(3));
/* I2C_SDA */
HWREG(SOC_CONTROL_REGS + CONTROL_CONF_UART_RXD(1)) =
(CONTROL_CONF_UART1_RXD_CONF_UART1_RXD_PUTYPESEL |
CONTROL_CONF_UART1_RXD_CONF_UART1_RXD_RXACTIVE |
CONTROL_CONF_UART1_RXD_CONF_UART1_RXD_SLEWCTRL |
CONTROL_CONF_MUXMODE(3));
To test the I2C output on your setup, you can enable the I2C test mode through the I2C_SYSTEST register on the I2C, and enter it into a test mode that produces a continuous clock signal in master mode.
-Justin