configs: am43xx: Migrate to CONFIG_SYS_MEM_TOP_HIDE from CFG_PRAM

configs: am43xx: Migrate to CONFIG_SYS_MEM_TOP_HIDE from CFG_PRAM

The TI AM43xx HS boards have been using CFG_PRAM to hide the top 64MB
firewalled DRAM memory from U-Boot. However, CFG_PRAM only prevents
U-Boot from relocating into that memory, but it is still open for access
for any other usage.

Therefore, migrate into using CONFIG_SYS_MEM_TOP_HIDE which reduces the
ram size itself ensuring that memory is not visible to U-Boot at all
(either for reloc, or general usage).

Signed-off-by: Beleswar Padhi <b-padhi@ti.com>