ARM: at91: move "select SRAM" under SOC_AT91SAM9 and SOC_SAMA5

To simply the PM config the CONFIG_AT91_SLOW_CLOCK option will be removed,
so move "select SRAM" from under AT91_SLOW_CLOCK, add "select SRAM if PM"
under SOC_AT91SAM9 and SOC_SAMA5

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Tested-by: Sylvain Rochet <sylvain.rochet@finsecur.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>