boards: arm: sam4s_xplained: Enable SRAM on SMC bus

This commit enables the SRAM controller is66wv51216dbll connected
to the SMC/EBI bus. Half of the memory is made available (512KB)
on chip select 0, the other part is (512KB) on chip select 1.

The SMC timings are taken from the Microchip studio example
SMC_SRAM_EXAMPLE for the sam4s_xplained board.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>