STM32 Flash fixes

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

This is a know hardware issue on some STM32 see the errata of your model
and if you make use of both memory banks you should enable it.

Writing to a flash sector while starting the erase of other sector
have a undefined behavior so lets add a semaphore and syncronize
access to Flash registers.

But for the semaphore to work it needs to be initialized so each
board needs call stm32_flash_initialize() on initialization, so
to avoid runtime problems it is only using semaphore and making
it thread safe if initialized, after all boards starts to call
stm32_flash_initialize() we can remove the boolean and the check.

Erase a sector from the second bank cause the bit 4 of SNB being set
but never unsed, so trying to erase a sector from the first bank
was acually eraseing a sector from the second bank.

Kconfig: Change the minimum SMP_NCPUS to 1

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

drivers/lcd/st7565.c: Use ST7565_POWERCTRL_INT instead of ST7565_POWERCTRL_BRF

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

The attached .patch implements DMA support for the stm32f4 I2C. Max and I have verified that it works on our systems.

WIP: Add support for Mikroelektronika Clicker 2 for STM32

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

sem_holder:Clean up from Review

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

Spelling and backward DEBUGASSERT along with one gem

  if (sem->holder[0].htcb != NULL || sem->holder[**1**].htcb != NULL)

XMC4xxx:  Clean up problems associated with USIC initialization.  USIC still does not work in UART mode.