Merged in jussi_kivilinna/nuttx/stm32l4_i2c_rewrite (pull request #519)

Port STM32F7 I2C driver to STM32L4

* arch/stm32l4: port STM32F7 I2C driver to STM32L4

    STM32L4 I2C driver is in work-in-progress state (plentiful of
    TODOs and #warnings) and lags many features found in more
    up-to-date STM32 I2C drivers. The peripheral on STM32F7 and
    STM32L4 are identical except for L4's 'wakeup from stop mode'
    flag and STM32F7's I2C driver is in more 'ready to use' state.

    Patch ports the STM32F7 I2C driver to STM32L4. The I2C clock
    configuration is kept the same as before (I2CCLK = PCLK1 80 Mhz)
    instead of switching to STM32F7 arch default that is I2CCLK=HSI.
    Further work would be to add configuration option for choosing
    I2C clock source instead of current hard-coded default.

* arch/arm/stm32f7: i2c: restore bus frequency after I2C reset

    Copy frequency restoration fix from STM32L4 I2C driver to STM32F7 I2C driver.

* arch/arm/stm32f7: i2c: remove unused Kconfig option

* configs/nucleo-l496zg/nsh: enable I2C4 bus with i2ctool

Approved-by: Gregory Nutt <gnutt@nuttx.org>