mtd: spi: Kconfig: Update CONFIG_SPI_FLASH

1) CONFIG_SPI_FLASH is not just a legacy code, but it has common
   core code which handle both dm and non-dm spi flash code. So
   fix the info text to make it clear globally.

2) Since it's flash core it shouldn't depends on legacy SPI,
   so remove the 'depends on SPI'

Cc: Vignesh R <vigneshr@ti.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>