arm: atmel: at91sam9x5: cleanup cs configure for spi

As the cs for spi is worked in gpio mode, so no need to configure
it as peripheral and then configure to gpio. Configure it to gpio
directly.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
arm: atmel: at91sam9x5: cleanup unneeded undef

remove unneeded #undef for at91sam9x5ek board.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
arm: atmel: at91sam9x5: move CONFIG_SYS_NO_FLASH to proper position

In config_cmd_default.h, it will use CONFIG_SYS_NO_FLASH to decide
whether include CONFIG_CMD_FLASH and CONFIG_CMD_IMLS. So, move the
CONFIG_SYS_NO_FLASH to proper position, then we don't need to undef
these two commands.

Signed-off-by: Bo Shen <voice.shen@atmel.com>