at91: rename mem_init() to at91_mem_init()

at91: rename mem_init() to at91_mem_init()

The AT91-based platforms have a mem_init() function declared in
arch/arm/mach-at91/include/mach/at91_common.h and implemented in various
places. In preparation of the introduction of the lwIP networking library
which also has a global mem_init() function, rename the AT91 one to
at91_mem_init().

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Hari Prasath Gujulan Elango <hari.prasathge@microchip.com>