dev: Fix the SPARC and X86 platform devices.

dev: Fix the SPARC and X86 platform devices.

A recent serial device refactoring changed the name of the parameter
that the terminal device gets attached to on the UART. The x86 and
SPARC platform devices didn't get updated though, and were still using
the old name. This change updates those objects.

Reported-by: Kanad Basu <kanad.kut@gmail.com>
Change-Id: I0824a9df8639062d8561420ea9ffea26b8b7e2e9
Reviewed-on: https://gem5-review.googlesource.com/5781
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>