soc: lowrisc: opentitan: Fix manifest sizing

The actual manifest is 1024 bytes, but we previously had padding bytes
due to the implementation of SECTION_FUNC(). The manifest is not
executable code so SECTION_VAR() is appropriate and produces a section of
the appropriate size.

Fixes: #82822

Signed-off-by: Shawn Nematbakhsh <shawn@rivosinc.com>