arch: Stop using TheISA:: in the ISA parser.
arch: Stop using TheISA:: in the ISA parser.

The ISA description provides us with a namespace to use, so we can use
that instead of TheISA.

Change-Id: If1a13c39a09ab22c7cab1138220fa0162fc2effa
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49707
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>

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