ARM: OMAP2+: Remove unnecesssary GPMC definitions and variable

With commit 21cc2bda (ARM: OMAP2+: Remove apollon board support) the
variable "boot_rom_space" is now not needed and the code surrounding
this variable can be cleaned up and simplified. Remove unnecessary
definitions and clean-up the comment as well.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Tested-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>