ARM: make memset and memcpy prompt message more clearly

The origin SPL_USE_ARCH_MEMSET/MEMCPY use same prompt message
as USE_ARCH_MEMSET/MEMCPY, which makes it's hard to distinguish
them in menuconfig interface. This patch gives them different
prompt messages for spl and none-spl config.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>