ARM: configs: Add new config fragment to change RAM start point

Rather than duplicate a defconfig for each difference
between platforms, we can choose to pick a basic defconfig and
manipulate it at run-time using config fragments.  Here we're
adding a new fragment to over-ride the RAM start point to 0x0.

Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>

arch/arm/configs/dram_0x00000000.config

+CONFIG_DRAM_BASE=0x00000000