arch: arm: aarch64: linker.ld: Fix ROM section names

This commit fixes the improper naming of the ROM sections.

1. Rename the first ROM section, which was previously named using the
  `_TEXT_SECTION_NAME` definition, to `rom_start`, as this section does
  not actually represent the text section.

2. Rename the second ROM section, which was previously named
  `_TEXT_SECTION_NAME_2` which supposedly refers to the definition of
  the same name that does not exist, to `_TEXT_SECTION_NAME`. Note that
  this is indeed the section that contains the text section from the
  source image.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>