spl: ram: hide SPL_RAM_SUPPORT

spl: ram: hide SPL_RAM_SUPPORT

Make SPL_RAM_SUPPORT a hidden Kconfig symbol, automatically selected
by SPL_RAM_DEVICE or SPL_DFU. Avoids the situation where SPL_RAM_SUPPORT
may be enabled without the other two being enabled, which results in the
following build warning:

common/spl/spl_ram.c:19:14: warning: ‘spl_ram_load_read’ defined but not used [-Wunused-function]
   19 | static ulong spl_ram_load_read(struct spl_load_info *load, ulong sector,
      |              ^~~~~~~~~~~~~~~~~

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>