Arm backend: Add PReLU decomposition pass

Decompose aten.prelu.default into clamp, mul, and add so the Arm
backend can lower it through existing TOSA-supported primitives.

Scalar PReLU weights are used directly; 1D per-channel weights are
reshaped for broadcasting over the channel dimension.

This intentionally uses the same lowering for U55 and U85 instead of a
U85-specific where/select path.

The pass only supports scalar or 1D PReLU weights, assumes per-channel
weights apply to dim 1, and relies on the existing
clamp/mul/add/view_copy backend and quantizer support for dtype and
shape coverage.

Signed-off-by: Per Held <per.held@arm.com>

Change-Id: Ib3889c27eb51b7aa21c99439b8edadffc57e0e13