The previous wording "top of PSRAM" was ambiguous: the carve-out is
actually mapped at the highest physical addresses of PSRAM (after the
rodata, text, and main heap mappings). Update the Kconfig help text for
SPIRAM_ENC_EXEMPT and SPIRAM_ENC_EXEMPT_SIZE, the External RAM
documentation, and the internal layout comment to say "upper end of PSRAM
(highest physical addresses)" instead.