spl: ram: Convert to CONFIG_IS_ENABLED

This patch is a preparation for adding TPL support for RAM loading.
CONFIG_IS_ENABLED allows for proper handling of the U-Boot/SPL/TPL
differences in config symbol names.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Tom Rini <trini@konsulko.com>